Kik is here, but there's a slight issue that we picked up on a while back when it was released. Being in the UK I attempted to use my .co.uk email address. Unfortunately Kik seems to have a slight issue with registering email addresses using this country code.
Fortunately, after much frustration, I used my GMail address and of course it worked. I don't really use my Google account for email so I decided to have a good look through the settings within the app and sure enough there's an option to change the account email address. Entering my .co.uk address seemed to work and I received an email from Kik confirming the change.
It seems as though it's limited to registration only, so use a .com email for signup and then simply make the change in the app settings. Job done! Kik have said they are working on the fix. Are you using a country code which is not .com and are experiencing the same issue? Let us know in the comments.