I had the same problem and, after scouring the internet in search of a solution, found nothing. So I started messing around with things. About 5 minutes ago I came up with something that worked for me! I'm still in 'testing' mode but it's playing some ACDC now so I'm happy. ;-)
My first step was to download a firmware 'update'. I'm not sure if it's newer or older than the firmware that I already had in there, but it works so what the heck. It's version 1.16 and is available for download here:
Filename is 'RioForgeFlash_116.exe'
To update the firmware, I connected the player via the USB cable. It will power itself up automatically - make sure you have a fresh set of batteries in it. Once it powers up, turn it off but leave it plugged in via USB. Then, power it up again but press and hold the mode button (the small button on the face, to the left of the main navigation buttons. It's the one you use to select different play modes etc.). It will power up and display 'UPGRADER'. From this mode, run the udate file (RioForgeFlash_116.exe); it will update the firmware in the Forge.
Once completed, my Forge interacted normally with the Rio Music Manager and I was able to upload music to it again. I did lose all the music that was in there and on the storage card, but that's better than having a useless MP3 player!
Best of luck!