MonoDevelop 2.8.2 / Unity 3.5 can't connect to iPhone 4S for debugging

I get this error when trying to attach the debugger to the process on my iPhone 4S:

"Could not open port for debugger. Another process may be using the port."

I've seen this issue in other forums (StackOverflow has a thread about it) with no solutions. I've checked and nothing on my Mac is using this port. Also some threads mention you can debug via USB, but I see no option for that in MonoDevelop 2.8.2. Where is that?