Here is a link I found to work from me. I am running Windows 7. I have VirtualBox installed and a copy of Ubuntu 11 something. I wanted to use terminal to install guest additions.

http://tinyurl.com/cfgnbk2

Basicly I ran two lines:

sudo apt-get install dkms
-A package will install itself. Won’t take long.

sudo apt-get install virtualbox-guest-additions
-Guest additions download and installs itself. This will take some time.