Okay, you need kernel header files to compile a lot of things, and certainly anything like drivers. They will likley be in one of the RPM files on your RedHat CD. Install that. You should end up with a bunch of stuff in /usr/src/linux/include (that's the standard place anyway).
After that, it should be able to compile the module, although I find it a bit wierd that nVidia don't supply a binary for RedHat!