ron-n-toni has it right.
Another way of putting it......
It works similarly to the two latches for the battery compartment. You have to unscrew it a lot to allow the latch-arm to align with the slot in the steel tube. You have to "feel" it go in the slot just like the battery compartment. Applying pressure in the center of the cover will flex it some to find the slot more quickly. But unlike the battery compartment latch, it requires many more turns to latch tightly.
One more point.....
Once on and tight, the folding handle must be in a particular position for the locking key to work. Tighten the handle all the way, then back-off to park the handle into it's proper position. If the key'd lock won't work, unscrew the handle a half-turn more and then the key will work.
Like you, I too struggled working the cover until I figured out how it works. Once you figure it out, it won't be a challenge again.