That message is saying the file /lib/modules/2.6.18-1.2798.fc6xen/build is pointing to a non-existent directory (or the link itself does not exist, possibly).
The "build" file is supposed to point to the source code for the kernel. If you are getting a file not found error there, the most likely problem is that you don't have the kernel source installed, which you will need to compile external modules like this driver.
If you do have the proper source installed, then the link might be missing or bad. You can check by running:
ls -l /lib/modules/`uname -r`/build
For example, running that command on my machine returns the following:
tj@T-Bird:~$ ls -l /lib/modules/`uname -r`/build
lrwxrwxrwx 1 root root 21 2006-06-19 20:55 /lib/modules/2.6.16/build -> /usr/src/linux-2.6.16/
Which indicates that the file itself exists, and that it is properly pointing to the location of my kernel's source code.