How do I use api.exchange.get_actuator value for a Setpoint Manager?

I read the documentation but I dont really understand what the parameters are and I can't find examples that explain it.

Have you looked at the EnergyPlus example files? Some are using the Python Plugin.

PythonPluginReplaceTraditionalManagers_LargeOffice.py is pretty much doing exactly what you need. The key is that you do not actuate the SetpointManager, you actually use the "System Node Setpoint" "Setpoint Temperature" actuator on the node itself. An alternative if you have a SetpointManager:Scheduled for eg is to actuate the Schedule Value instead.

The key part is:

Get the actuator handle:

handle = self.api.exchange.get_actuator_handle(
    state,
    "System Node Setpoint",
    "Temperature Setpoint",
    "VAV_5 Supply Equipment Outlet Node"
)

Set the actuator value to 50C, do that in the on_after_predictor_after_hvac_managers callback for example
```
self.api.exchange.set_actuator_value(state, handle, 50)
```

Study the example files, it should become clear.