MonoDevelop.VersionControl.Mercurial is a Mercurial addin for monodevelop.
The basic structure is copied from MonoDevelop.VersionControl.Git in monodevelop source tree.
MonoDevelop.Mercurial uses Mercurial.Net and Mercurial.Net uses command line hg client, so it is supposed to be in your PATH.
Currently it is built only with MonoDevelop master.
It is not very solid yet. Be careful to deal with non-referencing features such as commit and push.