dual channel memory

I have a question about the dual channel memory.
I have an ASUS A7N8X -X motherboard and also a big poblem.I don’t know how to activate my dual channel memory.When i start my computer, just before to start the boot section a mesage is displayed , informing me that the memory runs in single channel mode.the memory modules are identical(3 * 256 ddr ).When i start looking in bios the memory section is partialy blocked, and i don’t have acces to all subsections.