By going to game's Steam Store page, buying the game, then installing it using Steam. It's 33% off till December 1st right now.
What you see here is just free early prototype of the game.
If you own the game on Steam already, you'll need to visit the Radio Tower, Vasagatan and Balboa Island first. On Balboa Island you'll get code that allows you to get to Caravan Island, first of the two story locations introduced in Chapter 2 update.