Big orange button under the FILE MANAGEMENT tab.
See the above screenshot (I edited the post). Also, from the FAQ:
What happens once we go live...
If you're a user, meaning you don't author, you need to be patient for a little more. When we will go live with the final version, which is soon, you can just go to the c3universe.com login page and log in. You can find your password in your My Profile page here on the forums, it's called Unique token.
If you're a C3 original author, you will find all your files and stats from the beginning of our time (April 2013).
If you're an author, you need to first make sure you posted a song in the Customs database. If you have, when you log in you will be assigned the Unverified account. Request an upgrade from the File Manager and if we don't find anything wrong with your request you will be given the upgrade quickly.
Yet nothing about how to become an author...