Why is the P.O. manual payment option not visible at checkout?

We have established customers with net 30 accounts who purchase things from us using P.O.s We are now opening an online store where they no longer have to call us, they can just place the order online and we fulfill it. But we don’t want them to have to run a credit card at time of purchase, we want them to be able to just type in their P.O. number and we will bill them as usual.

I added a customized manual payment option, called it P.O. and activated it. But I still can’t see it as an option when I go to the checkout. Why is it not showing up as an option?