I have the similar issue here all the time... Clients want play a DVD during a Catalyst show or whatever. You have some ways to do it....

If you have a Blackmagic installed... you can use a conversion box from composite to SDI and use the SDI input. like told before here.

You can use either the Component input of your card or even use only the Y cable and that is a Composite input. to do that you also need to your System Preferences, Blackmagic and configure what Input are you using.
I am assuming you have a Blackmagic Decklink Extreme.

another way... easy one, is to buy any cheap capture box, what has a composite input and a Firewire DV output.
Catalyst reconises all those cheap boxes since they are Mac compatible.
I use all the time one PINACLE DV, is really cheap and to play a DVD or insert a DV camera does the job.

Is nece when you play a DVD heve only ONE layer active at the moment, or may be you experience a DELAY.

I also use that to leave a question, I tried before import a DVD to the computer, but playing from the Catalyst, the time of the movie changes from one play to the other... even with only one layer selected.
Is a way what i can tell Catalyst he needs to keep on the real movie time.
I know catalyst skips frames, and slow some movies to do not freeze, that is great feature, but is a way to play a movie and say to stay on the movie timeline or so. Even using a external Timecode source. Thanks

Gian