@CraigyD I tried adding a field and did all the things you have mentioned above, but no luck. Seems like something is stopping form customization, now problem is all the scripts for this electronic payment module is locked so cannot lookup what going on.

Ah, damn. Thank you for at least proving I'm not going mad here! I thought it was something to do with the payment file templating but I added references to the custom field there too and no change. The documentation is a bit patchy...

Try clearing your cache as this has been a common problem for me not having fields show up on forms.