So as you can see I figured out how to register a username (otherwise I wouldn't be able to post here). It wasn't very straightforward however.
I had to look through the page source to find out where to register. I read something about a link at the top of the page, which I could not find, and I checked the page source to be sure. I'm not sure if it is because of my web browser (latest firefox) or perhaps an abnormally long email address (it really isn't very long though), but the link to the 'get a username' bit wasn't visible in my browser.
Here is a screenshot of what I was seeing -> http://i.imgur.com/YzC39.png. If you need more info on my browser or whatever send me an email. Also if anyone else if having this issue and finds this post, the url to register a username is http://www.telltalegames.com/mytelltale/getausername/.
Hope this helps, also big thanks for making mac versions. Now that I can play these games at work, all you need to do is make official linux versions and I'll be able to play at home too :).