You will need to remove the front door to allow the tray to exit the computer properly. This is identical to the existing drive mechanisms installed in those systems. To do so, simply use a paperclip to manually eject the drive tray using the emergency eject hole on the front of the drive bezel, gently pull the drive tray open and turn it upside down.
Looking down onto the drive, release the locking tabs and slide the front of the tray DOWN toward the table. The front door is now off, and you may install the drive into your system. Please refer to: http://eshop.macsales.com/tech_center/index.cfm?page=removedrivedoor.html
Wren, just to clarify further...
Moving the jumper is critical. I forgot to first time around and both DVD drives wouldn't work.
With the tray extended, turn the un-installed drive over to release the bezel ; to work your flat head screw driver into then cracks to spring it loose. Take your time at this stage.
Move the Pioneer jumper on the drive to cable-select (middle position). Use a straightened paperclip to punch the tray out; put the clip into the tiny hole near the tray; this releases the tray. Carefully unclip the bezel from the front of the tray. Use a tiny flat head screwdriver to work it loose at one end, then carefully work it loose toward the middle and then the other end. The bezel must be removed so the tray can extend out of the Mac Pro. Otherwise it won't fit.
Now you're ready!
Follow the installation video at this URL: