The type or namespace name `Gtk' could not be found. Are you missing an assembly reference?

After fixing items below related to MonoDevelop on Linux Mint 7.3, another error shows up:
http://8thstring.blogspot.com/2016/01/monodevelop-does-not-start-on-linux.html
http://8thstring.blogspot.com/2016/01/could-not-obtain-c-compiler-c-compiler.html

The type or namespace name `Gtk' could not be found. Are you missing an assembly reference?

To fix install gtk-sharp-2.0 libraries, like:

$: sudo apt-get install gtk-sharp2