It's a single Windows executable that generates a .map file that can be imported, the only big pre-req should be having the .NET 2.0 framework installed.
Update: In case you don't care to read through the end of the thread, this tool now does normal staircases and has some support for basic railing/bannisters.
Update #2: I found myself wanting to make arcs and holes, so support for those are built in now as well (you can think of an arc like a stone arch, and a hole like a bevel + cap, except made out of brushes instead of patch meshes.)
Download
This post has been edited by Drewfin: 02 November 2010 - 04:32 PM