ScoreCleaner offers live scoring for Mac

Notating music doesn’t get simpler, faster or more musical than with ScoreCleaner. After opening the software, simply connect your MIDi instrument and start playing. The piece you’ve played is notated in the “listening window”. Double click on it and ScoreCleaner converts it into notation, automatically detecting time signature, key, tempo, polyphony, smallest note value, and much more.

Doesn’t get any easier than that.