wget http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el5.rf.x86_64.rpm
rpm --import http://apt.sw.be/RPM-GPG-KEY.dag.txt
rpm -K rpmforge-release-0.5.3-1.el5.rf.*.rpm
rpm -i rpmforge-release-0.5.3-1.el5.rf.*.rpm
yum --enablerepo rpmforge install dkms
yum install gcc make
yum install kernel-devel
Mount guest additions from the Virtualbox Devices menu (you will need to press right control to free your mouse pointer from the guest.
mkdir /media/cdrom
mount /dev/cdrom /media/cdrom
cd /media/cdrom
./VBoxLinuxAdditions.run