Hey I got ther same problem. You need to close media car and open up Mappoint and select the proper comm port in the tools menu. Then close mappoint and re open mediacar and click gps-->mappoint.
Then you shouldn't recieve that error. There may be conflicts with AGPS ans Mapoint though
AGPS complicates things. I hope you didn't pay money for it yet. There is another skinnable touchscreen friendly free software that does routing. It also accepts voice control
Check out this Need Testers for Mappoint embedded Navigation System
Download all of the files and place them in a single directory. Then link to this as an external gps with MediaCar. Fully skinnable. Nothing but good things to say about it. Although it is in an Alpha state it is highly functional