The Float Switch should hang a few inches above the top of your sump pump motor. The exact height will vary depending on the size of your sump pit and the power of your sump pump motor. The generalization is that you should hang the sensors at a height that allows for ample time to receive a notification and do something about the problem before a flood occurs.
Note: be sure to only use the float in your sump pit as the water detector is very sensitive and can trigger false alarms due to splashing in your pit. Use the water detector next to floor drains and under appliances where it is normally dry.