How do I play an NTSC dvd on my computer?

PAL/NTSC shouldn’t make any difference. The problem you’re running into is probably region coding. Give VLC a shot, or google for “region free dvd player mac”.

More info on Region codes here: http://en.wikipedia.org/wiki/DVD_region_codes

Right. Well actually, I think the problem might actually be the the discs are ‘HDVD-9’. Whatever that is. I think it’s some sort of compressed dvd format, but when I insert the disc it does not show up in finder, and gets ejected after a few seconds…

Never heard of HDVD9, and google isn’t much help. Try asking over in the Doom9 forums. If anyone knows, they would.

I know that VLC was previously mentioned, but perhaps it wasn’t clear that you should not run the bundled DVD Player and VLC at the same time. Also, you can change your system preferences so that when a DVD is inserted it launches VLC instead of DVD Player.app.

FYI region code “0” plays all regions, and region code “1” is for NTSC in the U.S.