I believe I saw you mention this in another thread and you indicated you have the steam version.  You need to do a few things.  First, purchase the game (which you obviously did).  Then, launch Demigod.  Click on Multiplayer.  Click on create account.  This will take you to a webpage.  There, you need to make a stardock account.  Then register your Demigod serial number to that account.  Once that is done, you should be able to login using the information from when you made the stardock account (that will be your user name and password).  Hope this helps.  If you are having other issues, please reply here with the specifics or shoot an email to support@stardock.com. 

I'm having the same problem as krow1012, I bought Demigod via Steam, downloaded, installed, made an account & registered the cd-key, then started the game up and tried to login to multiplayer: " You have entered an invalid username and/or password. " I've probably tried hundreds of times now, since I bought the game, but the username and password combo work on the stardock site, so I'm sure I don't have that wrong.
I have tried to change my password, and I have sent an email at support@stardock.com, but no help yet, just an open ticket.
A solution to this would be appriciated,
Thanks in advance.

Take a look here.  http://forums.demigodthegame.com/414332

Someone else appears to have found a solution - see towards the bottom.