This information applies only to a AISWatchMate 650. All other display models have built-in magnetic variation computations.
Check the setting "Course and heading mode" or "Bearing to target mode". If it is set to magnetic and you are still seeing your heading or target bearings in true, then it's because your GPS is not providing magnetic variation. Marine GPS units usually include this data in their RMC sentence, but embedded GPS modules, such as those used in transponders and other devices, do not provide magnetic variation.
If you have a transponder and you want magnetic headings or bearings then you will need to connect the AISWatchMate to another GPS. You can do this by using the second port on the AISWatchMate and selecting GPS as the input device for that port.