To setup an S300 in (Windows) follow these Instructions. You need to first have gotten the IP address from the S300.
Go to Configure
Go to Settings
Go to Payment Gateway
In the Dropdown select PAX. Put in the IP address of the PAX into the box followed by a colon symbol and port number 10009 . Check the box that says "Enable POSLINK"
Press the Green checkbox to save
Go back to the Payment Gateway screen to make sure the POSLINK box is still checked. Test by doing a 0.02 credit transaction.