Monk needs Git
The Monk executable needs Git in order to clone skeletons, which are Git repositories themselves.
This does not mean you have to use Git to version-control your project (although, of course, you’re highly encouraged to!).
If you need to install Git, follow the instructions found on the official website.
You can learn more about Git by reading Scott Chacon’s book or by watching the talk he gave at RailsConf ‘08.