It's a Korean 2-disc box set, with region code 3. I have found that I can successfully play it on the iMac with VLC, without having to change the region coding on the drive. delwyn had said that both the drive and DVD Player.app observe region coding, which may be the case, but VLC still operated.
Hints: When DVD Player automatically runs upon insertion, ignore the dialog box that appears. Clicking Cancel will eject the disc. Eventually, it will appear that the disc is not mounted. However, VLC's Open Disc function still finds it. This leaves one problem: if the disc is not mounted, how do you eject it? To avoid having to restart (so that you can run DVD Player and click Cancel), just open iTunes. It's Eject option works even if there's no audio CD in the drive. Yay!