Payment form fields can be used for the custom payment methods only. As for the other Payment Plugins, it's not available in sellacious yet. However, if you want this done in your site for other payment plugins too, you can order customization.
After creating the attributes in the payment form, you need to select them in the Custom Payment Method. You can do that from Sellacious Backend > Settings > Payment Methods. Select the Payment form fields.
If you're using more than 1 Custom payment method, kindly select those fields in all methods.
I hope this helps.