Click on Bankroll Boost (listed at the top left of the page) - on that page (3rd option on the right), you'll see an option to sign up for carbon. That link will take you to carbon's page to download the software and set up an account.

If you already have an account on carbon you can just go to carbonpoker.ag and download it - the option I mentioned above gets you opted in for our bankroll boosts (you get pts on the site for rake you accumulate).