The area that is going to be under the tile put a thin coating of hydraulic cement (smooth and level) but with a very slight texture (extra grip for thinset), this will water proof the concrete pool surface. Then you can use thinset to set your tiles. Use a modified thinset at Home Depot or Lowes. Use grey thinset if your plaster is going to be dark and a white if your plaster is going to be white. Make sure to leave room between the tiles for the plaster (you do not want to fill between the tiles with thinset, you want the plaster to fill between the tiles). Almost forgot hydraulic cement is called Water Stop at Home Depot or Lowes
Hope this helps.