It'll work like this
1) First become a founder. Yah!
2) Wait until alpha. Boooo!
3) When the alpha is released you will receive a CD-key from Stardock. Huzzah!
4) Use this cd-key to register the alpha on Steam. Anticipation!
5) Launch game and log in with your stardock account.
6) Enjoy the alpha! Hurrah!