It you are using a PAE (Physical Address Kernel) kernel in a Virtual Box guest and wish to install the VirtualBox Guest additions, you need to download the kernel-PAE-devel package as well as the kernel-headers package. PAE kernels are available for Fedora, Red Hat, CentOS and more.
Otherwise you will end up failing to build the main Guest Additions kernel module and getting a misleading message in /var/log/vboxadd-install.log after attempting to run sh ./VBoxLinuxAdditions.run:
*** Error: KERN_DIR does not point to a directory.
That message is misleading as the real error is due to you not having the PAE headers installed. For example, the PAE headers for the 3.9.10-100 Fedora 17 kernel should be located under /usr/src/kernels/3.9.10-100.fc17.i686.PAE – not under /usr/src/kernels/3.9.10-100.fc17.i686.
Unless you have deliberately installed the kernel headers in some unusual directory, you should normally never have to configure the KERN_DIR or KERN_INCL environmental variables.