I haven't used MapSource for ages but i seem to recall it doesn't play well with Marine Charts. Try installing HomePort instead. Both MS & HP are depreciated/unsupported by Garmin now and they've removed visible download links from their servers. This is the only one i've found for HP, it's version 2.25 but after install it'll offer to upgrade to latest V2.32:
Spoiler: Click
https://web.archive.org/web/20141129040306/http://gawisp.com/perry/homeport/HomePort_225.exe

After you copy and paste that link into a browser it might take a little time to offer the download window. Press the 'Go' button to the right of the EXE description if it doesn't start automatically.