Install VirtualBox Guest Additions

Launch your VBox VM from the VM VirtualBox Manager's main interface.

From the VM's window, select menu Devices > Insert Guest Additions CD image…
Then mount the CD:

> mount /dev/cdrom /media/cdrom
mount:/dev/sr0 is write-protected, mounting read-only

Install necessary packages:

> sudo apt-get install linux-headers-$(uname -r)
> sudo apt-get install build-essential linux-headers-`uname -r` dkms
> sudo /media/cdrom/VBoxLinuxAdditions.run