VirtualBox compiles kernel modules against the running kernel. This means you should update your system before you start as otherwise the Guest Additions will stop working when you upgrade in the future.

yum clean all
yum update

I recommend doing this in a virtual terminal of the guest machine (press CTRL-ALT-F2) as in graphical mode VirtualBox seems to have a habit of making the kernel of the host OS panic. Once you are updated, make sure to reboot so that you are actually running the latest kernel:

reboot

Now install the headers and compilers needed by VirtualBox Guest Additions:

yum install kernel-devel kernel-headers gcc

And install the guest additions by choosing ‘Install Guest Additions…’ from VirtualBox’s menus. Reboot again and your systems should come up with the guest additions working.