Open your ship and the mask. Double click  in ship pallette to turn it into layer, if not a layer already. Shift drag the mask over to the the ship image (clicking shift will put mask image on new layer exactly in the position of your ship layer) . On your mask layer go to the channels pallette. Control click the rgb icon. (Since your mask is just black and white, you could control click any of the channels.) You will see marching ants around your selection. Go back to your layers pallette. Go to your ship layer and click the add layer mask icon at bottom of pallette. Delete the top layer you used  (original mask) to create the mask. Ta DA!  If you need more help let me know.