Hey Eric,
You have to enable another setting to allow the user to setup a username and password.
On the Buy Now Settings page under
Sign In & Registration, you need to have enabled or checked
Show Sign In Page &
Require Login. A picture is shown for example.
If only Show Sign In Page is selected and Require Login is unchecked, it will not show the form fields for username and password.
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Once both of those are enabled, your Register and Checkout page should look like this if they are not logged in.
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Hope this helps