I am in the exact same situation as you.

I have pre-ordered Sins of a Solar Empire Rebellion on 07/04/12 from steam. Downloaded, installed the beta game through steam and created a Stardock account on the stardock.com website.

When I click to play the game a minimized window comes up titled Stardock Account Registration, enter my stardock account email and password then click register. Sometimes the window disappears before I can enter my details but when I enter them quick enough by pasting in my email first then typing my password in very quickly it shows this message "sins of a solar empire rebellion registration was successful"

So when I click to play the game again it shows the same minimized window titled Stardock Account Registration asking me to put in my details to register again when I already have ?

I submitted a ticket to support@stardock.com and someone dealing with my problem with no success yet.

If you do manage to solve your problem please message me back.

Good call on asking stardock,  I thought that since I was booting the game through steam I'd log a ticket with them.

They finally got back to me today (I think its like 6 days later?) with:

 I am still having the same problem booting.

I found out that my anti-virus software (avast) which I initially thought it was my firewall that was affecting the Stardock Account Registration window disappearance. So I deactivated anti-virus for the registration process and now this message shows in a new window on top of the registration:

Stardock Account Registration

[Symbol = black explanation with a yellow triangle background]  "Maximum activation reached"

So then I press ok and the click close or quit then another message shows:

[Symbol = white cross with a red circle background]  "An error has occurred while activating the product"

Which I know means I tried to register the game too many times but still don't make a difference.

By the way did you get a serial number with the game because I never?

Anyone having issues with product registration need to email support@stardock.com. They are the only ones who will be able to assist you.

Crap, that must be it for me then as well- Avast! I mean.  I just turned off all the realtime protection shields and tried to load it up and it worked this time.  I guess I didn't try to register as many time as you did.

Also, I believe you don't get a serial key until the game officially launches.  When it does you'll be able to right click the game in steam library and select view cd key to see it.

My game finally loads now too!  

Told Evan Technical Adviser about my new problem and he reset my activation limit.

Stupid avast anti-virus LOL