Follow these steps to install the Guest Additions on your Debian virtual machine:
* Login as root;
* Update your APT database with apt-get update;
* Install the latest security updates with apt-get upgrade;
* Install required packages with apt-get install build-essential module-assistant;
* Configure your system for building kernel modules by running m-a prepare;
* Click on Install Guest Additions... from the Devices menu.
* Run sh /media/cdrom/VBoxLinuxAdditions.run from the terminal (as root), and follow the instructions on screen.