/lib/modules/<version>/build should be a symlink to the kernel source for <version>, so you'll likely have to install that as well, and perhaps make the symlink yourself.
Code:
cd /lib/modules/2.6.18-elive
ln -s <path-to-2.6.18-elive-source> build
The ln command will likely have to be run as root, so either su or prepend sudo to the command depending on how elive does things.