Mercury Editor comes as a Rails Engine, but if you'd like to use Mercury outside of a typical Rails application, you can download these packages and use them.
Mercury expects a back end for many of its features, so you may have to duplicate some of that functionality in your own application. For information on how to do that you'll want to read the Using Mercury without Rails and Server Specification Wiki pages.
You can download more current files if you like living on the edge. These files represent where the project will be headed in the next release, and may or may not be stable.
Here's the older releases if you need them too.
We'll be adding the full walkthrough soon (promise!). But while you wait, you can watch an awesome RailsCast about getting it up and running in a Rails project.