@its_me's answer almost worked, but it failed building some kernel modules.
If it doesn't work for you, look at the messages.
In my case, one of them asked to look into
/var/log/vboxadd-install.log. There it said something about missing kernel sources and to provide the kernel source directory with e.g.
kernel-devel and sources at
/usr/src/kernels/2.6.32-504.16.2.el6.x86_64 were already installed, I was confused, but called it with the requested environment variable
KERN_DIR=/usr/src/kernels/2.6.32-504.16.2.el6.x86_64 sh /media/cdrom/VBoxLinuxAdditions.run
Now it complained about missing header files and suggested to do
yum install kernel-devel-2.6.32-504.el6.x86_64
As you can see, this is a slightly different version,
504.16.2 vs just
504. After installing the proper
kernel-devel package, building and then installing worked as expected.
So, running kernel and installed kernel sources (aka
kernel-devel) must match. You can verify this by looking at the running kernel with
and the installed sources at
As a final conclusion, whatever happens, look at the messages and log files provided.