Did you get what is called a SET chip with the machine? It is an eprom that you plug into the main system board (mpu board) and it runs a special program that allows you to enable the bill acceptor after you install it. If you didn't get one with the machine you will likely need to order one, they are not expensive.
They make a CLEAR chip also, they are not needed very often, you probably don't need to get one of those but can if you want to. It is used when you have a machine error and need to reset it back to factory defaults.
Someone else here more familiar with the S+ can tell you how to change the payout limit settings.