VirtualBox Guest Additions - Redhat / CentOS
Mount VirtualBox Additions disk through the VirtualBox UI.
yum install -y gcc kernel-devel-`uname -r`
mkdir /mnt/cdrom
mount -o ro -t iso9660 /dev/cdrom /mnt/cdrom
sh /mnt/cdrom/VBoxLinuxAdditions-amd64.run
Info found on tuxtraining.com