If you are running Ubuntu in Virtualbox and you need the Virtualbox Guest Additions to get better performance and higher resolutions, then type this command to install the required drivers. You will need the kernel headers installed to enable this command to complete sucessfully. But this works very well.
sudo apt-get install virtualbox-guest-x11
I tried this in Virtualbox 4.3.10 on an iMac and it worked perfectly with Ubuntu 13.04.