ddfczm is partly right. (Unfortunately) I started doing this project in XNA 3.1. MonoGame is a port (or reimplementation, technically) of XNA 4.0. That's where the differences stem from.
There's an XNA 4.0 branch on Github, where I've ported everything so that it compiles. It is out of date though, so you'd need to take the newest client version, compile it with XNA 4.0 and fix all the errors.
There are issues though, graphics bugs - which is why I didn't continue, IIRC. I can't remember what bugs there were though.
Main thing you need to know is XNA 4.0 doesn't support Targa image files natively, so you need TargaLib to load the files and convert them to PNG.