Missing Confirm button on registration

2Checkout is the current live payment gateway and the Confirm button doesn't appear after the account is created.

This article looks as if it will answer my question, but the solution was not posted:

I have also read the following posts:

with no solution.

I have installed the Twenty Twelve theme and uninstalled all plugins - the problem persists.

I suspect that I might not have configured 2CO correctly, but how do I test this without the Confirm button appearing?

I have also added this line to wp-config because the payment message did not make sense:

Any ideas? I'm stumped, after going through all of the documentation many times over.