VirtualBox comes with an "extras" iso image which allows you to run your guest operating system in full screen mode as well as get the best performance from it. However, Backtrack does not notice the CD drive on your virtual system, so we need to use a work around to get the installation scripts onto the machine.
From the virtual machine, click "Devices" on the top menu and then "Install Guest Additions". It will pop up asking you to download it. Click yes to this and wait for the download to complete. When it asks you if you would like to mount the image, click "No".
Now if you are running Linux on the host, you should be able to set up Samba in order to share with other computers on the network. If you open your file manager and ensure that you are showing hidden files, you should be able to find a ".Virtualbox" directory in your home directory; in here is the guest additions iso. Extract the iso using an archive manager; if you do not have one, 7zip is a good free one.
Once you have done that, share the .Virtualbox folder using Samba so the guest system can see it. Then go back to Backtrack and find the Samba share. Copy the additions folder you set up to your guest machine, open the terminal, and then go into the folder you just copied across.
You can now run "chmod +x ./autorun.sh" to make it executable and then "./autorun.sh" to install the additions.