For existing customers, you should be able to access the site with the same password as before and all purchases should be up to date. For new users, who have only made their purchases over the past 2 weeks, you may not yet be in the system. Firstly, try the lost password page to retrive your password. If that fails, just register yourself using the same email address as the one you used during purchase.
Once logged in, go to "My Account" page and you should be able to see all your purchase along with the latest download link.