Download the latest binaries for your platform to play the game. Currently, we have builds for Windows, OSX, and Linux! All free to play.
Before you start writing your own mission-packs, grab a copy of the docs. This is supplimental material to your standard Lua scripting, and should be all of the information you need to get started. As an added bonus, the default mission-pack is now in plain-text, making it easier to hack apart and learn from.