how do i get a dvd out of a computer in guest mode???

You don't, thats the entire point of guest mode. It prevents guests/students/customers/employees from stealing the disc.

You can force the computer to turn off holding the power button 10 seconds then hold the mouse key after the boot chime sounds. That will eject all the optical drives.

Depending on the login settings, you may not be able to log back into the guest account.