the directory you're looking for is a symlink to your kernel source directory, if it has red color and @ at the end, it means that the symlink is broken (you don't have the kernel source directory). So what you should do is install the kernel source (I see you're using the test kernel from Slack 10.2), just insert your Slackware CD 2, go into the testing directory and install the kernel-source packages. Do a 'ls -al /lib/modules/2.6.13/' and you'll see where the symlink is pointing, in your case it should be pointing to /usr/src/linux-2.6.13 . Try it, and post back with the result.
Right... simcox1 beat me to it