So you have MC21 running on your Mac and licensed? If so, good job.
You will want to tell MC which audio device to use. In this case, your external DAC. Go to:
Tools > Options > Audio > Audio Device
Pull down the menu and look for the name of your DAC. It should say the name and probably ends with (Core Audio). Select that, and click OK on the options panel. Now try playing music in MC and it should go to your DAC. If not, restart MC and try again.
JRemote: You need to turn on Media Network and generate and access key:
Tools > Options > Media Network > Use media network... > Checked
Tools > Options > Media Network > access Key > click it an generate one
You can then take that access key to JRemote and type it in so it can find MC on your Mac.
Good luck.
Brian.