The two ways of getting that effect is either to have the button light up when pressed or to have it shaded so that some part of it that is convex looked concave.
I think you mean the latter in this case, but the problem is that you have not constructed it to be shaded in a way to make it look physically pressed. So I think you could go for having it appear to be illuminated when depressed. Which would be a case of giving it some glow.
You have given it a drop shadow, so if the button were physically pressed down then that shadow would diminish so that would be one way to get the effect in this case. Also the type would slightly shift position.