Your site seems to timeout during the create account process so I cannot even test the checkout procedure.

Furthermore, index.php?main_page=checkout, is the correct redirect to the new fast and easy checkout. The admin configuration to turn on or off one-page checkout will have no affect on this and will merely reduce the fast and easy checkout process to only one page by removing the checkout confirmation.

To highlight this for you I will show each setting:

Normal Zen Cart Site:

Shopping Cart -> Checkout Shipping -> Checkout Payment -> Checkout Confirmation -> Checkout Success

FEC (Default = one-page off):

Shopping Cart -> Checkout -> Checkout Confirmation -> Checkout Success

FEC (one-page on):

Shopping Cart -> Checkout -> Checkout Success

What other mod's have you installed?