(I think there may be a tiny teeny weeny bit of a language barrier issue here!).

ss1122, in very simple terms, when you connect the 276C via USB to your PC, in order to send map sets to the unit, the easiest way to do it is through Mapsource (although transfer through other programs are possible).
You then use the selection tool (it looks like a polygon icon right beside the icon that looks like a magnifying glass) to click and drag over the charts you want.
The selected chart areas then go a purple to let you know there selected and ready to be transferred to your unit.

Then you click the icon that looks like a gps with an downward facing arrow ('send to device'), and mapsource will try and find your connected 276C unit.
A 'Send to device' window will open, with a 'Find device' button, and a small drop down box that should hopefully show Garmin 276C (or something similar).
If this box shows nothing, then check you have connected the Garmin correctly to your PC, or try clicking on the 'Find Device' button within the window to force it to detect the GPS unit.

If it finds it, you just click send and it sends it to the data card within the 276C, but please note it wipes any maps that are already in there. If you had a Nuvi, you would get a few selections within the drop down Device box (i.e internal memory and SD card), but for the 276C, there will only be one option.

You should have any maps you want to send already 'installed' to mapsource.

You can see what maps you have unlocked by selecting the 'Select a Product' drop down arrrow within Mapsource program, located just under the 'File Edit Find Transfer etc' menu bay.

If you dont see any maps here,or its blank, this means you have no maps installed on your PC/Mapsource program, and you would be right in saying you cant transfer any maps to the unit since you dont have any installed on the PC.

In that case, the only maps/charts would be the ones already on your 276C, which you cannot remove and put into the Mapsource program. It just doesnt work that way, you can only send maps to the 276C. You can recieve POI's and waypoints though.

You could get Charts and suitable maps from here if that was the case, install on to your PC and into Mapsource, using the various guides on this site, and then unlock to your 276C if you require them. Any charts or maps you install would then be visable in this Mapsource 'Select a Product' drop down box (irrespective if locked or not).

I'm not sure if the latest version of City Navigator is compatible with that unit anyway, but the older Marine Bluecharts (e.g. 2008) are perfect for the 276C.

Hopefully that helps, I know that a picture means a thousand words, but I dont have time at the moment to insert pictures of the steps above, I'll try at some other time to show you pictures of all the steps involved with my own 276C.

If this is not what the problem is, I apologise for the simplistic explination above, but if you could mabey explain a bit clearer, I'm sure there are loads out there that could help you if they understand what the problem is.

Good luck anyway