Login as root
$apt-get update
$apt-get upgrade
$apt-get install build-essential module-assistant
$m-a prepare;
Click on Guest Additions to mount CD-ROM
$bash /media/cdrom/VBoxLinuxAdditions.run

Technical notes, and other ideas.
Login as root
$apt-get update
$apt-get upgrade
$apt-get install build-essential module-assistant
$m-a prepare;
Click on Guest Additions to mount CD-ROM
$bash /media/cdrom/VBoxLinuxAdditions.run