1. Install dependencies:
$> sudo apt-get install dkms
$> sudo apt-get install build-essential
2. Reboot
$> sudo reboot
3. Menu bar -> Devices -> Install Guest Additions...
4. Mount Guest Additions ISO:
$> cd /media
$> sudo mkdir cdrom
$> sudo mount /dev/cdrom /media/cdrom
5. Install Guest Additions:
$> cd cdrom
$> sudo ./VBoxLinuxAdditions-x86.run
Hope that helped