Go to /lib/modules
See if 2.6.18-1.2798.fc6 folder is there. If it's not there, create
one, with the same exact name.
If it's there, check if there is a build folder. No folder, create
one. If there's a folder, you probably need to download some
compiling packages... the GCC and G++ packages.
Make sure you download the development files as well. Usually named
GCC-dev or something similar.