Is the output channel-strip showing signal at its' meter?
If not, focus on correctly assigning the channel-strips to the ganged output (obviously).
If yes, for a quick 'output-confirm' go to the very bottom of the File menu and try burning a "Share" to itunes. If the iTunes copy gives you output audio, then you know you have an output-assign issue.
My ad/da converter isn't recognized by my macbook pro/logic unless it is connected and on BEFORE I fire up my laptop. Might work for you.