I always have the same problem when I try to use my mirror modifier: nothing happens. It's very frustrating, and simply can't find how to fix this. Any ideas?
This is because you set the wrong mirror axis.
On your screenshot, we see that it's the Y axis you want to mirror (as the center point is on the edge which is on the side of the green arrow: Y axis)
So, try to set mirror axis to "Y" instead of "X".