Go off-line.
Select the B17 and look in the drop-down skins menu.
Count the number of user skins, don't count the default skin. There are two if I remember correctly so the next one you add would be 3.
Exit AH.
In your skins folder in AH2, create a folder named b17g# where # is the next skin count, probably 3 as in b17g3.
Put the three files you down-loaded into this folder; b17g1.bmp, b17g2.bmp and b17g.txt.
Launch AH again, go off-line and select the skin in the hanger. For best viewing you must use 1024 video textures but remember to change back before you fly the MA.
I think this is all covered at ahskins.