Follow these steps to install the Guest Additions on your Ubuntu virtual machine:
Login as ubuntu;
Click on Applications/System/Terminal (or on Applications/Terminal, if you are using the 6.06.1 Dapper Drake release);
Update your APT database with sudo apt-get update, and typing your password, if requested;
Install the latest security updates with sudo apt-get upgrade;
Install required packages with sudo apt-get install build-essential module-assistant;
Configure your system for building kernel modules by running sudo m-a prepare;
Click on Install Guest Additions… from the Devices menu, then choose to browse the content of the CD when requested.
Run sudo sh /media/cdrom/VBoxLinuxAdditions.run, and follow the instructions on screen.
