I'm pretty sure (although really would like it confirmed), that both the addPayment() and the pay() functions are unimplemented or unsupported. There's a couple of references to this on G+ too.
After adding a payment to an order using /v3/merchants/{mId}/orders/{oId}/payments I have to wait between 5 and 30 seconds for the payment to appear in the order. This is obviously not suitable for a live environment, is there a way to speed this up or ask the Register app to sync itself after a payment has been posted?