I have done very, very little with monodevelop myself, but I have looked in to it quite a bit (and have some bookmarks). The treeview widget is what you'll be using.
The old tried and true tutorial is http://scentric.net/tutorial/
, however, it's not in mono.
Here's some information from the mono documentation: http://www.go-mono.com/docs/index.aspx?link=T%3AGtk.TreeView
And here's a pretty nice tutorial which will show just about exactly what you're trying to do: http://www.mono-project.com/GtkSharp_TreeView_Tutorial