How to add an X-Board to WISE-7901?

Please refer to WISE-7901 Datasheet, section Wiring & Boarding for detail information.

Where can I find FRnet wiring information?

For complete details about FRnet, please refer to http://www.icpdas.com/products/Remote_IO/frnet/frnet_list.htm. Find your FRnet module manual and from the Address Setting section , you can find how to set up address for the FRnet module, to connect each FRnet module, please connect the AB port for each FRnet module in series.