First step is to make sure your howl works. Forget about mt-daapd, and just get howl to advertise mt-daapd. If you can’t get that going, there is more fundamental problems at work.
Edit your config file to advertise _daap._tcp on port 3689. See if that shows up in iTunes.
Then, start mt-daapd with -m. That disables the built-in responder. Once that’s running, see if you can connect. (let it run a while to make sure the database is indexed).
If you can’t get to this point, then it’s likely a firewall or other issue. So start there.