Not the true solution, but the bug is that really idiotic design is at work here...
in file,
/common_files/modules/XPayments_Connector/xpc_iframe.tpl
find:
<div class="button-hider"> </div>
and comment it out. The submit button will now be shown.
If x-cart cn fix x-payments, you can unhide this.
Unfortunately, the concept of hiding the submit button until all fields are populated may be a good idea, but if the damn thing doesn't work, it's a useless feature.
And it doesn't work.