I had the same problem , i fixed it by downloading the latest drivers for the serial-usb cable. Mine also showed the data coming in on maxsea
but no fix. After the driver update it worked 100% (use a TRENDnet Su-S9 cable and driver and win 7) Also remember to disable the "Microsoft serial
mouse" in device manager (that also causes the mouse pointer to jump around the screen
if its not disabled)
When you change the gps sentence type remember to press ok at both boxes to go out to maxsea main screen or your new settings wont apply fully.
When you start maxsea hold in eigther Ctrl or shift (cant remember) and it will change your settings to the default working settings.
Also check the last box to see if you have not got any checksum errors
If any of these do not work post a screen shot of your incoming sentences