P6td deluxe enabling all to dual channel ram?

Hi, I recently had ram die in my computer, so I went to the store and to my surprise didn't have any triple channel memory in the size that I wanted so I got all dual channel memory. 24gigs of it, to take up all the slots the guy said it would all work.

My Problem is it's only recognizing 16GB and not 24GB so it's essentially only recognizing the ram in the dual channel slots, I was wondering if someone could help me with the settings in the bios to get it to recognize all the slots as dual channel. Thanks.