well, when you import it into RMXP, you will have the option there and then to choose which colour is transparent. Just click with the right AND left buttons on the white area and it'll set it to transparent.
However, when you're viewing the tileset while creating your map, it WILL show with a white background in the left panel. If it still shows with a white background when you place tiles on the map (especially if you're using one of the two top layers) then it hasn't got the transparencies set correctly.