Mango Blog future plans include a way to add custom editors in a manner that is simple and compartmentalized, but this is not yet realized. Therefore, in order to accomplish this goal today, we must edit a few files. Don't worry, it's not difficult and there aren't that many to do.
Of course there are two pieces to this puzzle. First, you have to put a Markdown editor in your blog (directions to follow); then you need a plugin to convert that Markdown into HTML at display-time. I've written a plugin called Salsa to do that.
Download wmd-Mango.zip. Unzip it into {Mango}/admin/assets/editors/
so that the directory structure follows: