It's hard to explain, and photos may be difficult to show, but here goes:

The tray mounts to the bottom of the fuselage with the front edge bolted to the wing mount block. When you have the fuselage upside down, the tray (which MUST have the receiver and battery attached with rubber bands and completely connected to servos and switches) is inserted upside down into the rear of the fuselage. This makes it right side up when the fuselage is right side up. There are 2 small screws that are used to hold the tray in place.

It's all spelled out pretty well in the manual. Follow it to the letter. It may look like a jig saw puzzle, but it really does all fit, eventually

Just wait until you want to remove the battery!

The tray is located there to give all the tail weight possible, and you'll probably need to add more unless you plan to use a very light engine.

Just read the manual and do everything they say to do. There's no room for making modifications here.