The original release of WCAT 6.3 contained a bug that was causing many users to not be able to use the tool succesfully.
The bug resulted in the "NULL controller specified" error.
The perf team folks were nice enough to provide a fix for this bug, and re-release the tool.
You can find the updated bits for the tool at the normal WCAT 6.3 download locations:
1) WCAT 6.3 x86
2) WCAT 6.3 x64
You can find my original announcement about the WCAT 6.3 release here. You can find detailed documentation in the download, and also attached to my announcement.
As promised, I'll post some more info about using WCAT just as soon as I get the time soon.
As always, post here or on the other post if you run into any issues.