I'm assuming you know about and how to use SDC to manage your software. Is this so? If not you need to go to Stardock's main web page and locate their free download of SDC (Stardock Central). Once you confirm your registration (There's a button on the bottom right marked Registration Info, click it, Choose games, in Product toggle down to SoSE, input the serial number you were given, click on update). Your copy of SoSE should then be registered. You use the same serial number to create an Ironclad account.  

That method worked for me although it did take a couple of tries. When you click on the create a new account button it should prompt you for a user ID, try a different one in case the one you tried is already in use, a password and then when you submit it should, if I recall right prompt you for the CD key which is the serial number you were sent in the email. Use copy and paste(clipboard) to get it right. If not, I think the server may be having some type of issue perhaps?  

Yes, the serial is the same as the cdkey. If you're getting ICO disconnects, make sure that you don't have a firewall popup in the background, and that your router is not blocking port 6000.

ICO was acting up this morning. I got several disconnects. I don't know if it's gotten better as I have not played yet since then.