Assuming this is an UM VM (or, for that matter, any other Ubuntu derivative)
I always do the following:
Inside the VM, open a terminal and execute the following command:
sudo apt-get install dkms
Close the terminal
Insert the guest additions CD image via the Virtualbox “devices” menu
Cancel and close any dialog boxes asking if you want to run anything. Then open Caja in the VM and manually navigate to the Vbox Guest Additions CD
Once in there right click to open in a terminal
In the terminal that you have just opened, type “ls” to list its contents. You should see the following file:
VBoxLinuxAdditions.run
Execute the following command in the terminal:
sudo sh ./VBoxLinuxAdditions.run
When it has finished installing, execute the following command:
sudo reboot
Following the reboot, you should find it is now fullscreen