Now that you’ve clicked on the “Apply to Become Author” and the “I’m serious!” buttons, you need to either log out and log back into C3U, or just click on the “File Management” tab. There you will see a new pop up with some terms you must agree to for using C3U. Once that’s out of the way, you’ll see a button on the far right of the screen (you may need to widen your browser before it appears) letting you know your account is “maxed out” and to request an upgrade. That will be your first request for space. Once approved, you’ll be able to start uploading your customs.
Feel free to ask here again if anything seems not to work.