Could be as simply as right option-Clicking the Speaker icon in the taskbar and making sure the FW card is selected for audio output. Does it show up in the Audio MIDI Setup pane? [it's in the Utilities folder of your applications directory] You can right click any Audio I/O device to assign it as your default input/output... from there you have to assign channels your computer output to channels on the DM, ie "stereo Left to channel 31, stereo Right to channel 32" as shown in this screenshot.
That's one way of doing it, probably the most common. You could also just run your line out from the Mac into the 2TR input on the back of the DM and bypass FW altogether, though you're still going to have to configure FW to play nice with your audio applications.