..empeg has stated before that they might have problems when plugged in alongside another usb device - try just using the empeg, without your usb mouse, and see if it shows up... If you can, you should also try a different cable - you never know if it might have gotten stepped on/whatever.. As a last resort, downgrade your empeg/emplode software to what you know worked, and see if it still works..
-mark

...proud to have one of the first Mark I units