The manual RAM setting are set in the "AI Tweeker" section of your BIOS, which is second from the left in the main section of the advanced BIOS. To get into the BIOS, hold down the DELETE key while your computer posts (that weird beep you hear when you turn on the computer cold).
Also, I suggest you FLASH your BIOS to the updated current one, that usually means one in the 1000 series (don't load a Beta version!). You get the BIOS update from the ASUS website. This will help with your RAM issue and solve some other little bugs. If you are not comfortable doing this yourself, don't. Get a friend or pay a Tech to do it for you. If you screw it up, you could brick your board. I also suggest you read some of the other posts on this forum regarding RAM troubles and Sandy Bridge boards.
Regarding your specific RAM....is it on the QVL (Qualified Vendors' List)? If not, it may cause you grief. If it isn't on the list, I recommend you return it and buy RAM that is. The QVL is also located on the ASUS site under the downloads tab for your particular board.
I hope this helps you. Good luck and all the best.