Hi scott,
Some of the upcoming tournaments (assuming you're looking at the all active tournament list) are future events and are not yet underway. That's why you are having trouble entering them. The active list shows current and future events.
If you go to the Tournaments main page and scroll about halfway down you'll see the Tournament Calendar there. Those are all of the tournaments and CTTH competitions that are currently running. As an Amateur you may be restricted from entering a few events (like the Pro Tier open), but you'll be able to enter pretty much any other event there.
Good Luck =)