I'm assuming you're trying to run the LDK setup task in Fusion, but that is designed if you actually purchased the LDK kit which comes with 2 leak sensors. In your case with just a single leak sensor, you'll need to do it manually:
1. Plug in the leak sensor to the FMM
2. Go into the FMM configuration screen to ensure the FMM detected the leak sensor properly. If it did not detect it, be sure the plug is fully seated in the socket. It should be listed as "Water On Floor" as shown in my FMM ports 3 & 4 below:
3. Go to your list of Inputs, scroll through to find the leak sensor. It will be listed as a "Switch" with a default name based on the FMM's AquaBus address and the port number. In my example screenshot above, I'd look for Swx4_3 since my FMM is on address 4 and I have a leak sensor in port 4.
4. Click that Input to go to its configuration page where you can rename it to something more meaningful. Input names are limited to 6 characters, so abbreviate if needed:
5. Go back to your Fusion dashboard, click the padlock icon to unlock the dashboard and reveal the Unused Tiles window at the top of the screen. Scroll right to left until you find the new leak sensor's tile, then drag it down onto your dashboard. Click the padlock icon again when done.
6. Add a line of code to any output you want to turn off or on if a leak is detected. If the leak sensor detects water, it will report closed:
If LkSump CLOSED Then OFF
7. I'd recommend adding a line of code to your EmailAlm output so you receive an alert if a leak is detected:
If LkSump CLOSED Then ON
See my tutorial on Apex Alarm Programming if you need assistance in fine-tuning your alarms:
https://www.reef2reef.com/ams/neptune-apex-programming-tutorials-part-6-alarms.707/
Also, be aware that simple condensation on the leak sensor can trigger a false alarm. Many people place the leak sensors on a piece of paper towel to avoid these false alarms. A true leak will saturate the paper towel and still trigger a true leak alarm.
Hope this helps!