Our Sidejack family of guitars is one of the most successful of our catalogue, and as such is one that we receive a large portion of customer feedback on. One of the most frequent subjects that comes up from our customers has to do with the tremolo system on the guitar - "how do I seat the arm properly?"
The mechanism is designed in such a way that the arm needs to be pushed in as far as it physically can in order to function properly. Once pushed in, the arm will be able to swing 360 degrees around, yet still be snug enough that it's not going to pop out without you saying so.
Basically, you just need to push the arm in until you hear a satisfying "pop" sound. It may require a little extra force than you'd expect - but be careful not to overdo it. You want the "pop" sound to come from the tremolo unit, not your wrist!
Try as you might, if you can't seem to get it to seat, you may need to loosen the grips on the inside. It's a simple task - just unscrew the plate that the trem is attached to and you'll see a 4-tabbed grip that holds the arm in place. If the arm isn't going in all the way, you'll need to expand these tabs by using a pair of needle nose pliers and gently pulling each one outward slightly. Check the arm's fit with each adjustment so you don't go too far. If the problem you were having was the arm just falling right into the socket, and slipping out too easily, then you'll of course have to tighten these tabs instead.
That's it! From there, you should be able to insert and remove your tremolo arm without much difficulty, and have the comfort of knowing it won't fall out the next time you're jumping up and down on stage.