Go to Stardock Central. Go to games. Under purchased games, it should have GalCiv2. Select GalCiv2, look at the drop down that appears and you should see Dark Avatar. Right click Dark Avatar, select Register Products. In the screen that pops up, click the request serial number button. It will be emailed instantly. Check your email, copy and past the code into the serial number area. Make sure you change the product from Games subscription to Dark Avatar (I had this problem). Then press update, and you should be able to download it.

I also did not recieve any email. This method worked for me.

EDIT - Kryo beat me to it