To activate the fullscreen mode in a virtualbox machine, we need to install Guest additions in the guest OS. If you are testing Ubuntu 10.10 Maverick meerkat in virtualBox , you may notice that guest additions are not working, it seems that the problem is the new xserver they put in ubuntu10.10, the modules are not build against this new version (See screenshot bellow). To fix this issue i used the methode bellow to install Guest additions in Ubuntu 10.10 Maverick Meerkat, this methode consist on installing a new xserver display packages which works well.