Doesn't look like the instructions are available online that I can see (not even at the link provided to the product above).
It's been a couple years since I installed mine. If I remember correctly, you basically have to loosen the bolts on the hinge-side of the carrier, lift the arm up, tighten the bolts back down, and check the contact between the arm and the latch. Repeat until you get the arm and latch to meet while under whatever load you're planning to carry.
Adjusting the arm without the tire or jerry cans does you no good, because the arm will sag the second you put any kind of weight on it and immediately throw off your work.
I have a 33x12.50 Duratrac spare tire and two jerry cans on my carrier. I was able to adjust it so that the arm meets the latch pretty much spot-on.