Slitherine

I haven't been playing multiplayer in a while so when I tried to give it a go again I'm getting this message: "This serial number has not been registered. Client should open the registration/account creation screen."

What I'm supposed to do? Where do I find this registration/account creation screen?

Login to My Page (top of this page).
You should find all your serial numbers there. Earlier you had to manually register a game, but I think the purchase prcess now enters it automatically.
If you do not find it under My Page, send an email to support.