You can't submit a problem (riddle or any other type) until you've graduated from Novice to Student. You do this by rating at least one puzzle and posting three comments on puzzles on the site. It probably requires an overnight wait after you've done these things for the new rank to take effect. Once it does, there will be a link on the Home page near the login area in the upper right portion of the page, to click on to submit puzzles.