First of all, make sure that you've followed the installation tutorial here.
Go in your equipment properties:
In the selected properties, be sure that you've checked the one called "Encoded downlink message"
NOTE: It's the only property with a "Write" Access type
Now that all the requirements are completed, you can send the downlink. Go to "Live data" and click on "Set property"
1) Fill this blank with the value of your downlink. Downlink specifications are explained in sensor manufacturer documentation.
2) Once you click on "Save" the downlink will be sent to the sensor.