Oh these are a pain to remove.
under the sink, feel for a knurled nut on the back of the tailpiece that comes down form the sink it will have a lever activated rod coming out of it. Remove this knurled nut, it may be difficult, squeeze the little spring that captures the rod to close the drain. Now there should be a rod with a ball and gasket still inside where you removed the knurled nut, pull these out and the plug should be able to be removed. These units are known for the white plastic gaskets breaking, and the rod where the white ball is for rusting through BTW. I had to buy a new unit just for those parts.