Thanks for the quick reply.
So it seems like the problem really was the wrong VID/PID programmed. I used FTProg to re-program it, and after that it initialized just fine in both CCS v4 and v5. Not sure why it wouldn't work properly with MProg, but I'm just glad it's working now!
I upgraded to CCS v5, but I had a hell of a time trying to get anything to work. Most of the DSP examples provided in CCSv5 are for C2000 and C6000. I think I found a few for 550xEZDSP and 553xEZDSP, but nothing helpful for 5515EZDSP. I tried migrating v4 examples to v5, but I couldn't get anything to build without errors, so eventually I gave up and switched back to CCS v4.
As far as I can tell, CCS v4 seems totally compatible with windows 8. The proper drivers were installed when I plugged in the device, and I've been able to load and run multiple examples programs into it.
Thanks again!