I'm sorry but I can't help.
I have asked many times
http://www.subsim.com/radioroom/showthread.php?t=83987
how to add the heavy falk to the type IXD u-boat, as I love the conning tower #1 (use it only for all my ix u-boats), and I saw pictures of IXd2s with an heavy flak gun on the deck, just as the IXB.
By the way, how were you able to add the heavy flak to the IXD2 and IXC??
P.S. Did you notice that if you select the conning tower #1 for the IXD2 it says, in the weapons menu (the one before starting a new patrol):
one deck gun, and
one heavy flak gun
but then the option to add the heavy flak is not available?