I also have a 276C, also as a motorcycle gps, and I would solve your problem with the following methods.
If you are not to use the address search you can split the .img file with gmaptool and the install it to your pc (this is from a windows pc horizon, with Mac your hands are often strapped to your back unfortunately). When installed the map or preferred part of map can be sent to the gps with Mapinstall or Mapsource if it is an older map. Anyway the map can be seen with both Mapsource and Basecamp for the use with route planning and sending routes to the gps.
If the .img file you have can not be seen in Basecamp even when it is in folder garmin in an usb stick the problem cuold be that the flag Ms is not set to 0. This can be made with Gmaptool, write and set flag Ms to 0.
If you can see the map in Basecamp from usb stick (not installed in pc), now fixed with Ms-flag, can you send it directly from Basecamp/Mapinstall? I have not done this so I do not know if it works this way. Then it would be installed in gps with full address search abilities I think.
Sorry for the somewhat unstructured advices, I was just thinking of the possibilties you have. Anyway, the only way I know of how to get maps into the 276C is with the help of Mapinstall or Mapsource. And when sending maps to 276C, use external power in order to avoid problems with corrupt maps.
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