Select Alerts under the Flights tab. You must be logged in and belong to a subscription plan. In the upper-left hand corner of the screen you can setup a new alert. Please enter the airline, flight number, and date. Press the Setup New Alert button. Choose the type of messages you want to receive, where you want the messages sent and press the save button.
Alternatively, if you are using the flight tracker on a flight that has not landed yet, there will be a Setup Flight Alert button on the flight tracker screen. Pressing that button will start the process.