In order to open or close a pop-up drain, you move a lever attached to the drain. This will allow the drain to pop-up so water can flow through it. To take this drain off, use the lever to pop the drain up as far as it will go. Pull out the stopper and the arm. Using a wire brush or even a toothbrush, clean out all the debris inside.
If there isn't that much gunk inside the drain, then your blockage may be caused by something else. Take off the cover plate (the one with the lever on it)by loosening its screws. Pull out the drain assembly. Dip your toothbrush into some white vinegar and brush all the scale off of it. Using vaseline, lubricate all the pieces, and reassemble.