I've had this problem on a couple of versions of Trio, too.
Paul was emailing my beta builds just before 1.6 to test the html interface and in a couple of those builds the applet was broke. The applet was working with 1.6 up until last week when I installed java 1.5_02. That broke everything Trio at first. Now, I have disabled 1.5 for applications and the server and the html interface is working, but the appliet is not. I have seen no way to force applets to revert to java 1.4.

I set the java console up and it spits out these errors when I click on "player"

INFO: Remote exception while getting icon - server gone down?
java.rmi.UnmarshalException: error unmarshalling return; nested exception is:
java.io.InvalidClassException: javax.swing.ImageIcon$AccessibleImageIcon; local class incompatible: stream classdesc serialVersionUID = -3994512798706967451, local class serialVersionUID = -1903458698681874148
at sun.rmi.server.UnicastRef.invoke(Unknown Source)
at trio.player.TRioPlayerApp_Stub.getIcon(Unknown Source)
at trio.applet.TRioAppletPlayerPanel.getIcon(Unknown Source)
at trio.applet.TRioAppletPlayerPanel.playbackUpdate(Unknown Source)
at trio.applet.TRioAppletPlayerPanel.init(Unknown Source)
at trio.applet.TRioAppletPlayerPanel.<init>(Unknown Source)
at trio.applet.TRioApplet.getUnitApplicationPanel(Unknown Source)
at trio.applet.TRioApplet$2.stateChanged(Unknown Source)
at javax.swing.JTabbedPane.fireStateChanged(Unknown Source)
at javax.swing.JTabbedPane$ModelListener.stateChanged(Unknown Source)
at javax.swing.DefaultSingleSelectionModel.fireStateChanged(Unknown Source)
at javax.swing.DefaultSingleSelectionModel.setSelectedIndex(Unknown Source)
at javax.swing.JTabbedPane.setSelectedIndexImpl(Unknown Source)
at javax.swing.JTabbedPane.setSelectedIndex(Unknown Source)
at javax.swing.plaf.basic.BasicTabbedPaneUI$Handler.mousePressed(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Caused by: java.io.InvalidClassException: javax.swing.ImageIcon$AccessibleImageIcon; local class incompatible: stream classdesc serialVersionUID = -3994512798706967451, local class serialVersionUID = -1903458698681874148
at java.io.ObjectStreamClass.initNonProxy(Unknown Source)
at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
at java.io.ObjectInputStream.defaultReadObject(Unknown Source)
at javax.swing.ImageIcon.readObject(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at sun.rmi.server.UnicastRef.unmarshalValue(Unknown Source)
... 33 more
Exception in thread "AWT-EventQueue-2" java.lang.NullPointerException
at trio.applet.TRioAppletPlayerPanel.playbackUpdate(Unknown Source)
at trio.applet.TRioAppletPlayerPanel.init(Unknown Source)
at trio.applet.TRioAppletPlayerPanel.<init>(Unknown Source)
at trio.applet.TRioApplet.getUnitApplicationPanel(Unknown Source)
at trio.applet.TRioApplet$2.stateChanged(Unknown Source)
at javax.swing.JTabbedPane.fireStateChanged(Unknown Source)
at javax.swing.JTabbedPane$ModelListener.stateChanged(Unknown Source)
at javax.swing.DefaultSingleSelectionModel.fireStateChanged(Unknown Source)
at javax.swing.DefaultSingleSelectionModel.setSelectedIndex(Unknown Source)
at javax.swing.JTabbedPane.setSelectedIndexImpl(Unknown Source)
at javax.swing.JTabbedPane.setSelectedIndex(Unknown Source)
at javax.swing.plaf.basic.BasicTabbedPaneUI$Handler.mousePressed(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)