Goto virtual box linux downloads page http://www.virtualbox.org/wiki/Linux_Downloads and download the .deb file that is suitable for your processor (x86 or 64bit) and the version of your ubuntu.
Right click on the downloaded .deb file and choose "Open with GDebi Package Installer"
Click on the Install Package button. At this point it will install virtual on your ubuntu machine.
Now an menu entry will be added to Applications->System Tools->Sun VirtualBox
If the menu entry doesn't exists, just logout and login back, it will showing up correctly.