Install VirtualBox Guest Additions for CentOS 6

1) Make sure the computer has network connection
2) mkdir rpm
3) cd rpm

Location of RPMForge for CentOS: http://wiki.centos.org/AdditionalResources/Repositories/RPMForge
Download the rpmforge-release package. Choose one of the two links below, selecting to match your host's architecture. If you are unsure of which one to use you can check your architecture with the command uname -i

i686 http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el6.rf.i686.rpm
x86_64 http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el6.rf.x86_64.rpm

4) wget http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el6.rf.i686.rpm
5) rpm --import http://apt.sw.be/RPM-GPG-KEY.dag.txt
6) rpm -i rpmforge-release-0.5.3-1.el6.rf.i686.rpm
7) yum install dkms
8) y
9) Devices -> Insert Guest Additions CD image...
10) mount -t iso9660 /dev/dvd1 /media
11) cd /media
12) sh ./VBoxLinuxAdditions.run
13) It will complain of missing kernel-devel-x..., do a yum install kernel-devel-x... whatever version it is.
14) sh ./VBoxLinuxAdditions.run

Now you can use commands such as, vboxmanage guestcontrol