Are you wanting to watch DVD movies (as in a disc that can be purchased in a shop), or watch videos that have been burnt to a recordable disc on a PC (for example)?
With my Samsung SE-S084D external slimline DVD drive, I can not get Android to recognize the device is attached if I leave the drive in normal mode. However, if I switch to "AV Compatible Mode", by ejecting the tray, then pressing the eject button again twice in a row, it works fine, after about 1 or 2 minutes of an automatic mounting procedure (displayed as a notification).
Check if your drive has a different mode. I only ever noticed mine did because I unknowingly/accidentally switched modes and looked strange on a Windows PC.
You shouldn't need add-ons, as it is a USB device, and unless you've got a weird file structure/formatting system on your DVD, it should be fine. I should point out that in XBMC I could only directly play the VOB files of a DVD, it didn't seem to recognize the DVD video as anything other than a collection of files.
Ah... it is possible that DVD Movie playback capability requires a license - see the thread here -
http://www.raspberrypi.org/forums/viewtopic.php?f=35&t=17877
However, make sure Android can see the drive (via ES File Explorer or your preferred app), before shelling out on that.