That metal arm should swing out when the lid is opened. And I think maybe you have to push it back in manually after closing the lid but I can't remember. If you try pulling it out, it ought to pop out of there. If it doesn't have a spring attached to it, maybe that's why it's not popping out. I guess you could try a drop of sewing machine oil on that ball thing that's in the hinge.
I have never looked at how those arm things work. I've never encountered one that didn't pop out. Some that don't want to stay in is the more common problem.
bbl...