How to download the game
After you have pre-ordered the game you can now download the game by going to the secret pre-order forum which is only available to those who have downloaded the game. From there you can now find the latest alpha posting and download it from there. There you should see a thread named Unofficial Overgrowth a[XXX], weekly build. Just look for the first post in this thread and you should find download links.
Overgrowth Tutorial - Getting Overgrowth (Mac & Windows) - YouTube video by Silverfish