Why are all of the other payment methods unavailable?

I would like to attach a few forms to my BluePay account. But when I try to select it, all of the options but PayPal are grayed out. How do I do it?

You can only have 1 payment field per form, thus explaining why everything else is disabled except PAYPAL (which is what's currently used on your form).

If you wish to use a different payment processor, you have to delete the other one first. So, just delete the PAYPAL payment field, then add BLUEPAY afterwards.

Thanks Jim, but how do I delete the PayPal field?

Please find the section where the products are displayed or the section where you are showing up a total. Once there click on  the payment section and then on the trash icon to delete the payment field: 

I hope this helps.