First of all you need to have a fully installed copy of Ubuntu 9.04 (or any of it's variants) in VirtualBox. If you do not, please complete that before continuing. I recommend also letting your guest Ubu do a full update if possible before moving on.
Save a snapshot now before attempting to install Guest Additions!
Once you have installed your guest Ubu and saved a snapshot of the virgin install, you'll need to first release your mouse by pressing the Host key, then go to your Devices menu and select "Install Guest Additions..." This will mount the ISO for you under /media/cdrom0.
Open up a terminal, or switch out of X Windows to a tty and log in. Go ahead and
It should run through the install and ask you to reboot when it's done.
If you get the warning message above ("...unknown version of ... X Windows...") you may encounter problems after rebooting. If so, please read this post for help.