Thanks for getting back to me.
Just to be clear -- I should be able to directly connect the MIDI controller to the 3.5mm input, right?
The controller is an M-audio Keystation 49e. It's probably seven or eight years old and is now discontinued. It works fine with Garageband and whatever my son uses on his PC. I also ordered the Rock Band 3 keytar ($30) because I read that it's a decent controller and it would be a good base for an "on-the-go" synth platform. Plus, it gives me an opportunity to test another MIDI controller inexpensively,
I've ordered a new MIDI cable to test whether that is the problem. I might have to solder my own DIN to stereo. I found only one DIN 5 to 3.5mm stereo cable, but it's wired incorrectly, using pins 2,3,5 instead of 2,4,5. (see below).
Incorrect wiring from kenable: