From this site (For Ubuntu Server):
http://en.ig.ma/notebook/2012/virtualbox-guest-additions-on-ubuntu-server
1. Mount the CD Rom
2. Install a few important Kernel headers
$ sudo apt-get install -y dkms build-essential linux-headers-generic linux-headers-$(uname -r)
3. Build the Guest Additions
sudo /media/cdrom/VBoxLinuxAdditions.run
or
sudo rcvboxadd setup
4. Reboot