I have this occasionally. First, though, make sure that you have configured the medianet xml file with the IP address of the machine running Medianet. Then make sure that in the devices folder, you've edited the XML file under the RIO folder to ENABLE = yes (or something like that). Service must be stopped to edit these properly. Once these edits are made, restart the server. If you're running Windows XP and using a firewall, make sure that that the medianet executable is allowed to get through. Once those checks are all made and server is started, unplug your receivers for a few seconds and then plug them back in. Then power up the receiver. It SHOULD connect right away but sometimes I've found that I have to have the medianet server restarting as I am powering up the receivers.
If it doesn't connect, it should list and IP address and subnet on the receiver display. Make sure that the IP address shown is one that would appear on your network... meaning if the IP address shown is 126.96.36.199 or something like that but your DHCP server should be assigning IP's at 192.168.1.xxx then you have a network issue into the player. It will never find the Medianet server. If the IP addresses are fine, play with starting the receivers and medianet service. Sometimes when I'm messing with things it finds them instantly and other times it doesn't... But I can always get it to work