The steps to prepare a CentOS guest for installing the VirtualBox Guest Additions are outlined here, but since it references a few other pre-req steps, here’s a list of all the steps I took to get all the dependencies installed before you can install the Guest Additions:
Install yum-priorities (http://wiki.centos.org/PackageManagement/Yum/Priorities):
yum install yum-priorities
Install RPMForge (http://wiki.centos.org/AdditionalResources/Repositories/RPMForge):
wget http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm rpm --import http://apt.sw.be/RPM-GPG-KEY.dag.txt rpm -i rpmforge-release-0.5.2-2.el6.rf.*.rpm
Install dkms:
yum --enablerepo rpmforge install dkms
Install dev tools and kernel development:
yum groupinstall "Development Tools" yum install kernel-devel