ModSlaveSim help v3.04 - 3.3. Simulation: Edit Slave Registers
Download manual: HTML
This is where you add or remove registers for a Modbus slave and define how their values should change in the simulation.
To navigate to these settings, type Ctrl-W to bring up the Edit Settings... dialog then with the Simulation tab selected, click on the appropriate subpanel title under Edit Slave Registers..