Rim = the Metal cylinder that slips on the axel.
Wheel = Rubber either Tubed or Tubeless.
I am not a mechanical engineer, but my Dad is. He said the key is already in place and the Rim and Wheel should pull off the axel after you have removed the C-Lock washer and the washer under the Rubber cap.
Mine was so rusted that we could not get the rim off. You can try hitting the center axel in the middle of the rim with a hammer to get it to move but not too much or too hard because if you mushroom the end you will never get the rim off the axel without some more work. Anyway he was saying that the "KEY" is already in the slot. The Rim is notched and the axel is notched and when you pull the rim and wheel off the key should fall out or be there to be removed. You then put the key in when you put the Rim/Wheel back on. The "KEY" locks the Rim in place with the axel.
Anyway my problem was my Tubeless WHEEL was off the Rim but it was inside the Rim. The funny thing was he just pulled the Wheel toward the lip of the Rim while I added Air and the dang Wheel just POPPED right into place. It was brillant! I had spent hours on that thing and he fixes it in about 15 minutes. I finished mowing with any issues. He suggested getting some "Slime" a brand of hole seelant and use that. I haven't done it yet as I just got done mowing.
I was confused about the "KEY" as well and I hope this helps.
Engineer's Son, October 2008