I don't have an 'eject' button on top of my desktop screen

You should have a eject button on your keyboard. You only get the eject icon when your mac pro has two dvd drives.

In the finder you have a small arrow-type thing on the right side of any mounted disk you can eject. Pressing there ejects the disc. Also, you can take a DVD drive and move it into the trash. In the case of a DVD (or CD) this will also eject the disc.

As mentioned by the poster above, a Mac keyboard also has an eject button on the top right (little triangle).

If your keyboard has no Eject ⏏ key, hold F12 for a couple of seconds to eject.

If you want the Eject ⏏ in the menu bar, double-click:
/System/Library/CoreServices/Menu Extras/Eject.menu

To get rid of it, Command-Drag it from the menu bar.

You DON'T need two cd/dvd drives at all !

Just follow this path in finder.

System/Library/CoreServices/Menu Extras and double click on Eject.menu and it will appear in your menu bar !