It's a function of the plotter and the NMEA interface. The Oceanus only has a gps in interface. You can't get it to show mmsi's on your plotter. Don't feel too bad, I installed an Icom 304 and discovered it wouldn't do it, so I replaced it with a Uniden 525 which works fine with my garmin plotters (3210/3010). Before you scrap the radio, check to see if your plotter will support mmsi's on the screen. If not, you'll have to change the plotter as well.
Tiara 3700 Open/Cummins 450Cs