If you have already pre-ordered, you can download the Beta here.
For those who want to pre-order the game and also try out the Beta, you can do so here. (Note: for Windows.)
This an early Beta release. This is a way to offer people who have pre-ordered a sneak peek into the game, as well as an opportunity for them to offer feedback on features they think would be useful, awesome, and so on, if incorporated into the game.