If your tiles are arranged in a grid and won't be moving around or changing shape along the X and Y axis, you should be able to determine which tile was clicked on by simply dividing by the size of the tiles (and acounting for the map offset).
; 2d example const map_origin_x = 383, map_origin_y = 389 const map_tile_width = 32, map_tile_height = 32 ;;; tile_x = floor((mousex()-map_origin_x) / map_tile_width) tile_y = floor((mousey()-map_origin_y) / map_tile_height)
; 3d example const map_origin_x = 0, map_origin_y = 0 const map_tile_width = 32, map_tile_height = 32 entitypickmode(map_entity, 2) ; 2=polygon ;;; mouseover_entity = camerapick(global_camera, mousex(), mousey()) if mouseover_entity = map_entity then tile_x = floor((pickedx()-map_origin_x) / map_tile_width) tile_y = floor((pickedy()-map_origin_y) / map_tile_height) endif