Installation error VBOXGuestAdditions in VirtualBox Fedora 12/13

# cat /var/log/messages
Use above to look for reason for error messages.

# yum install kernel-devel kernel-headers gcc dkms
# export KERN_DIR = /usr/src/kernel/

dkms is needed if you get the modprobe error for guest
Now install VboxLinuxAdditions

If in spite of all the above it still does not work, then you may have a kernel versions mismatch.