You can see a demonstration of how to request powerlevel reports and how to handle the response in my Zwave Tweaker DTH.
Note, polling the powerlevel of a Z-Wave isn’t going to tell you much though. The powerlevel indicates the power the device is using to transmit commands, not the signal strength of received commands. It’s a feature designed for testing only. In normal operation the powerlevel is NormalPower. It will only be different from this if you have sent a command to tell the device to use a different power level. Any change in power level only persists for a short period of time (timeout period) before returning to NormalPower.