SmartDown (in common with all of my ExpressionEngine add-ons) is no longer officially supported.
Check out the wiki for installation and usage instructions, and feel free to fork the repo if you'd like to make some changes or improvements (it's distributed under a liberal open source license).
Hopefully this will be everything you need to use this add-on in your projects, but if not please don't email me asking for support; I don't even have ExpressionEngine installed locally any more.