There was a product design change that replaced the triangular tray with a round version.
Here is the correct way to mount it:
1. Carefully remove the plastic bolt from the accessory tray.
2. Spread the tripod legs apart as far as they go and place the tray on the top of the leg brace assembly.
3. Take the plastic bolt and thread it in from the bottom of the leg brace assembly and in through the accessory tray that is mounted on the top of the leg brace assembly.
You will see that the tray is being pulled down making the legs more stable and the tray more secure. It also has a built in hand control holder, eliminating the separate hand control hook on the leg.