Thanks. I downloaded the "kernel-source-2.6.13-15.i586.rpm" and installed it with YaST, it does it all automatically.
Anyhow, I found the kernel.h in "/usr/src/linux-2.6.13-15/include/linux" and linked it with "-kernel-source-/usr/src/linux-2.6.13-15/include/linux" as the installer said, but then I got another error:
ERROR: ./nvsound/main/conftest.sh: line 9: cc: command not found
If you are using a Linux 2.4 kernel, please make sure
you either have configured kernel sources matching your
kernel or the correct set of kernel headers installed
on your system.
If you are using a Linux 2.6 kernel, please make sure
you have configured kernel sources matching your kernel
installed on your system. If you specified a separate
output directory using either the "KBUILD_OUTPUT" or
the "O" KBUILD parameter, make sure to specify this
directory with the SYSOUT environment variable or with
the appropriate nvidia-installer command line option.
Uhm, how do I do that then?