Hi,
VirtualBox has an online version of its User Manual avaiable here
http://www.virtualbox.org/manual/UserManual.html
Quoted from that:
*Deleting virtual machines* (has to be done first)
To remove a virtual machine which you no longer need, right-click on it in the list of virtual machines in the main window and select "Delete" from the context menu that comes up. All settings for that machine will be lost.
The "Delete" menu item is disabled while a machine is in "Saved" state. To delete such a machine, discard the saved state first by pressing on the "Discard" button.
However, any hard disk images attached to the machine will be kept; you can delete those separately using the Virtual Media Manager; see the section called “The Virtual Media Manager”.
You cannot delete a machine which has snapshots or is in a saved state, so you must discard these first.
Uninstallation
To uninstall VirtualBox, open the disk image (dmg) file again and double-click on the uninstall icon contained therein.
(Quote end).
I presume that you no longer have the .dmg file from which VirtualBox was installed, so simply download the VirtualBox .dmg file from here
http://www.virtualbox.org/wiki/Downloads (click on the link at "VirtualBox for OSX hosts") and once downloaded double-click the dmg file and follow the a.m. instructions.
Hope it helps
Stefan