Now that you have posted your question, if you look at the bottom of the page you will see a section titled Similar Threads. There should be one or two that are relevant. There should be one titled
http://www.laptopgpsworld.com/762-problems-sending-maps-mapsource-garmin-mobile-pc which might help. Also, when you are in that discussion thread look again at the bottom of any page in that thread and see what the Similar Threads list suggests.
By the way, you didn't say which versions of Mapsource and Mobile PC you are using. There are at least a few versions of Mapsource that refuse to see Mobile PC no matter what you do.
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