Step 1. Put the Ohmmeter in the OhmsX1 mode (low Ohms). See what it reads with the leads open (my Fluke says "OL"). See what is says with the leads shorted (my Fluke says 00.4Ω).
Step 2. Find the two terminals (three possible pairs) that have near zeroΩ with the switch closed. Try both positions of the switch handle, so six total measurements. Note which two terminals appear to be shorted with the switch closed. One is the "IN" terminal (in my diagram), the other is the "load" terminal, but up to now, we don't know which is which.
Step 3. Put the Ohmmeter in the diode test mode (->|-). See what it reads with the leads open (my Fluke says ".OL"). See what is says with the leads shorted (my Fluke says .000).
Step 4. To find which of the terminals found in step 2 is "load", first make sure the switch is turned off. Connect the Red Ohmmeter lead to either of the terminals found in step 2. Connect the Black lead to the third terminal "GND" (which is neither "IN" or "load"). If no conduction ( .OL), move the Red lead to the second of the two found in step 2. If you see some conduction (about 1.700 on my Fluke, that is approximately Vf of the LED), then the second one of the original pair is "load", and by default, the other one is "IN".