I had the same problem. I'm using win 7 64 bit. sonar 8 producer 64 bit. The Microsoft GS Wavetable synth does not appear anywhere as it used to in XP/VIsta. On both my desktop and laptop running the same thing, I needed to change the audio setting in sonar to Driver Mode MME (32 bit). Then start the wave profiler. when finished, close and restart sonar and the wavetable appears.

It doesn't matter on my desktop but I like to use my laptop with a mini midi keyboard away from home and now it works fine!

Hope this helps someone.. and WIN7 is the best I've yet seen. It's pretty much approaching the MAC!!

Kev