Getting the action button is a little complex. I figured it out, based on Gods Of War, and I meant to write up a tutorial. But I never got around to it.
The code is in the Fury Road mod, see my sig or the download database. Briefly, you want to do the following. (Fury Road filenames in parentheses)
1. Define the unit you want to perform this, make sure you can control it in the game using WB
2. Define MOUSE_OVER help text for the action button. (XML/text/FuryRoadTextObjects.xml)
3. Define button art and create a related button (XML/art/CIV4ArtDefines_Interface.xml, search for RESETTLE)
4. Add code to draw the button, optionally disabled (Python/EntryPoints/CvMainInterface.py, search for "DaveA" comments)
5. Add the connection between the button from #3 and the help from #2 in getWidgetHelp (Python/CvGameUtils.py, search for RESETTLE)
6. Add code to call your local action routine (Python/EntryPoints/CvMainInterface.py, handleInput)
7. Add code in the local action routine to actually perform the operation (Python/CvFuryRoadEvents.py, handleResettle)
If you are good at python, this may be enough, but I should probably add a tutorial with a lot more detail.