issues memory

Hello, I also need your help if possible! I have an Asus p8z68-v lx motherboard and I'm trying to put ram on it, model cmx8gx3m4a1600c9 xms3 Corsair 8 gb each, two boards, but instead of 16 gb it only shows me 4 gb in the bios. Did I do something wrong or do I need to activate something? Thank...