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, then run mount /media/cdrom.
- Run sh /media/cdrom/ , and follow the instructions on screen.