Ok, this is a step by step walkthrough of how to download the beta if you have already pre-ordered it for Sins of a Solar Empire. This may or may not be required, but after spending 10 frantic minutes I got thoroughly annoyed.

1) Open up Stardock Central.

2) Click on the Games tab on the right.

3) Locate Sins of a Solar Empire in the game list.

4) Highlight and right click on it and select Register Products.

5) In the new menu, select Sins of a Solar Empire in the drop down list.

6) Copy and paste the serial number provided in the e-mail that was sent to you today in the Serial Number text box.

7) Click the Register button.
(Sins of a Solar Empire should now be under Purchased Software)
(Make sure you check the Show Pre-release Versions box on the bottom.)

8) Highlight Sins of a Solar Empire, an click the Install button that appears.