Screwed
BigG nailed it...That ain't a screw.
To remove the catch, push it in like he said while exerting very light pressure in a counter-clockwise direction. When it hits the spot, it'll turn easily. When it stops turnin', push the button in farther and it'll slide right out the right side of the frame.
Sometimes the sweet spot is a little tough to find. Slowly work the button in and out while ya keep pressure on the screw head. To reinstall, just find the spot again, and turn lightly in a clockwise direction. The catch will seat with a "snap."
All ya wanna do is feel for a rough or sharp edge and dress if a little. It's also possible that the catch spring is a little stiff, and it's just hard to seat the mag. Go ahead and force it past the tight place a few times and it'll loosen up.
Luck!
Tuner