A friend at work gave me a GeForce 9600GT today so I could try out CUDA. Like many of you, I had some problems getting it to work. I'm an IT guy so troubleshooting wasn't all that tough. Here are some common things to check.
First, make sure you have everything in the CUDA checklist installed properly. Even if you think you did it, do it again.
Second, try the "whereismycuda" application. You should be able to extract it to a folder and run it. It doesn't like version 3.1 because it's looking for specific DLLs... I don't know if Nebula also looks for the 3.0 DLLs but I know that the tester will work if you change the file names to what it's looking for. If you're positive that you have the CUDA toolkit installed but it's complaining that it can't find a dll file, you might have the path set wrong. Look at your CUDA paths in the system environmental variables: System Properties, Advanced Tab, Environmental Variables, System Variables. You'll have three. Make sure that these folders exist and are populated with the right files. If not, change them and reboot. (Or try it without rebooting... I don't know if it will matter.)
An easy way to test whether CUDA is working and your paths are wrong is to copy whereismycuda.exe to the BIN folder within C:\CUDA and then run it from there. The application looks locally before checking the environmental variables so if it works, it means that your computer's CUDA is setup and you have some sort of version or path problem. If not, check your drivers and installation of the bridge.
Make sure that the version of the toolkit and bridge you install matches the version of Nebula you're using... which should match the version of the DAW. I'm running Windows 7 Ultimate x64 but a 32-bit Cubase 5. My drivers are 64-bit to match my OS but Nebula matches Cubase so CUDA needs to match Nebula. In my case, the problem was that I installed the 64-bit toolkit first and it set my environmental variables to the 64-bit paths. In the end, I simply copied C:\CUDA\bin and renamed it C:\CUDA\bin64 because changing the environmental variables wasn't working. I knew that it was using the path so rather than change it, I just gave it the folder it wanted. CUDA now works in Cubase.