How to play a dvd movie with external subtitles?
This is a temporary answer as I don't have a Mac at hand now. 

I have to test on a Mac, but the Windows version of 5kPlayer can do it very well; and it has a Mac version.

Also, KMPlayer can also do it (also tested just in Windows for the moment) and it has a Mac version it seems (here - Mac Alpha Version: have yet to test too).