First if you have it plugged in and seemingly running, check your desktop preferences to see if it is recognized already.
Go to the Apple Menu > About this Mac, click the More Info button and check what devices are on your USB section of Profiler. If your DVD drive is showing up there, it is ready to use. However, nothing will show up on your desktop until you insert a disc to read.
Perhaps you have a disc in and it doesn't show up. Go to Finder > Preferences > General and see if the CDs, DVDs, and iPods option is checked.