After a lot of searching and trying many solutions, I got it finally:
uninstall connector
uninstall MySQL for Visual Studio from control panel
reinstall them according to the table below
copy the assembly files from C:\Program Files (x86)\MySQL\MySQL Connector Net 6.9.8\Assemblies\v4.5
to C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE
log off and reopen your solution
enjoy