I think you are not looking at the right "GPS" menu. In the Maemo Mapper main menu you open first the "Settings". (If I recall correctly... I do not have the device with me right now to check.) That opens a window with many different settings... and one submenu in there should be GPS. In that submenu you should be able to tell Maemo Mapper what kind of GPS you have. There you should select the first option, which should be something like "Bluetooth device". Select that and leave the address field empty.

After saving the settings you should close the menu and then you should select the GPS from the main menu and enable it. Hopefully this helps.