As it's been posted before on this site, and on various others (i.e. gamewinners.com) You must collect ALL of the other skulls, then preferably clear away the flood for safe transit in between the rings. If you get the sequence correct, the rings will pulse and the IWHBYD Skull will appear before the light bridge connecting to the Ark Activation Panel (where Truth's corpse lies). This must also be done on Normal or higher.
On a side note, only one question mark is needed to end a question. Grammar is key.
First, the edit button is useful. We have had too many problems with double posting, it needs to stop.
Second, the skulls all appear on the Normal difficulty. I would recommend watching videos posted in the forums addressing this issue directly to see exactly where the skull is.
Mods, please close this thread, as it is kind of redundant