"This Apple ID is valid but is not an iCloud account" Trying to sign into iCloud on my Macbook and I get this error message. I've also had some trouble signing into the same account with iCloud on iPad and iPhone, but the messages there are even more vague.

My Apple ID is a gmail address - not anything from iTools/Mobile Me and existed as an Apple ID before iCloud was released.
Go into System Preferences » Users and Groups » and then under your name on your login it says Apple ID. Put your details in there and that should sort out your problem. Well, at least it did for me.