|You should not need to configure anything on the router as it will be the credit card machine initiating all translations. |
As far as routers are concerned, that is an important concept to understand. What that means is the the credit card company will not be able to access the credit card machine unless it firsts initiates the transaction. Network Address Translation within the router will prevent it.
However, there may be a requirement for the Credit Card Company to be able to access the machine for administrative purposes, in which case you will need to set up port forwarding.
You should need to do is to ensure that the credit card machine is on the same sub-net as the router, otherwise it will not be able to contact anything. This can either be done via DHCP if the machine can act as a DHCP client or you may have to configure it manually,
The best thing to do is to read the instructions that comes with the machine, that should tell you everything you need to know If you don't understand IP addressing, DHCP and NAT you would probably be better getting someone who dose.