You still haven't installed the kernel-source-$(uname -r) package, have you.
You NEED this package installed before the NVIDIA_kernel package will compile!
If you have, then for some reason, there is no build symlink in /lib/modules/$(uname -r), and there is no /usr/src/linux symlink either. You'll need one of those, and it's going to have to point at wherever the kernel sources have been installed to (usually /usr/src/linux-<version>). If that directory doesn't exist, then you haven't installed kernel-source.