I'm trying to hook up my new Lowrance HDS7 gen3 to my laptop for live data. I have the same set-up with a HDS5 gen1 and it works great. I have gone through the output for NMEA 0183 and have all the channels set the same as the gen1 unit. What I'm I doing wrong?

Hard to say. Are you able to set up logging on your PC COM port, to make sure that data is coming through? That's the first thing I would check.