The 'n' stands for native. As in recording only the native frames to 30p (or 24p) instead of recording all frames into a 60i stream and flagging the frames to be ignored or removed by the NLE. For now Media 100 cannot deal with 720p30n. Really the only work around is to cut in another NLE or run the footage thru Compressor to bring it back to 59.94 (which is 30p with 2:2 pulldown).
The major advantage to shooting native is that you can get more footage on your cards since you are only recording those native frames.