Please remove installation media and close tray message inside Virtual Box
Just to be clear: you can just hit enter when you see this message after installing inside Virtual Box and Ubuntu will restart normally.
Make sure the VM is set to boot the hard disk you installed Ubuntu to first, then restart.
The message is designed to make sure that users don't accidently start the installer a second time. The easiest way of making absolutely sure that doesn't happen is having them remove the CD from the drive.