That's part of the ModBus protocol, every slave has an address and reacts only if the master is requesting information from that address. The Google Drive files you linked to contains only files for the master part of the game. You also need the slave libraries and there you set the address, usually in the code but you can also choose the store it in the EEPROM and read it at every startup.
So i found the Simpel modbus project and installed the examples. This works perfect. I can talk between two arduinos using the potion meter.
Reading that I would expect that you already installed a master and a slave part and you're already using an address to specify the slave you want to query. From the slave library there exists a newer version, please update.