You have entered the stardock.net account information that serial is associated with right? You may have to enter your friend's e-mail address and then use that serial for it to work. Not totally sure since you have a strange situation.
Evilidler, if you have tokens on your TG.Net account you can go to http://www.totalgaming.net/ and purchase the game with your tokens by logging in there with your StarDock.Net account. They will then send you your serial key etc and download instructions. That has seemed to work for some other people that were having trouble finding the release version on SDC.
Martimus, One thing you can try is go to http://www.totalgaming.net/ and login to your account there. Then click on "Buy" for Sins of a Solar Empire and you can "purchase" the game with your tokens on that window (assuming they show up there). They will then send you an e-mail with your serial key / download link etc. I've read that others are having trouble using tokens to purchase Sins right now as well.
Hey think my friend had the same issue and I figured out how to get it to work (you will need the serial eventually though I think). Go to TotalGaming.Net and in the top of the window, you should see where you can login to a TG.Net account. Instead click on "Create Account". Create a TG.Net account here using the SAME e-mail address and information as your StarDock Store account (they are 2 totally different accounts turns out!). Then once you do that and verify the TG.Net accoutn vi