If you haven't been able to sync the wallet since updating, you probably need to start it with the -reindex option.
That is quite strange. Normally it would use a wallet.dat file in that directory, unless you otherwise specify a different data directory or wallet file, so I'm not sure how you're seeing a balance if you don't even have a wallet.dat file in %appdata%\myriadcoin...
Anyway, to reindex you can just start the program from the cmd window using "myriadcoin-qt -reindex", or you can make a shortcut to the qt wallet, and right click and go into properties of shortcut and add " -reindex" to the end of the target line, and then launch from shortcut.