I have a P8Z68-V Deluxe, I imagine it's the same thing. First you have to change the SATA mode to RAID, but I assume you've already done that since you have a RAID0.
I'm going to bite the bullet here and admit a really stupid mistake that I made. When I was trying to set up my RAID1 I couldn't get ctrl+i to work. I spent several hours on the phone with ASUS and I even RMA'd my first board. It turned out that I just wasn't pressing ctrl+i exactly the right way. I was pressing them at the same time as my computer booted but you have to press ctrl *then* i within a split second of each other, but with ctrl distinctly first. If you tap ctrl+i simultaneously as the computer boots it won't work. If you hold ctrl and tap i it won't work. What I had to do was repeatedly tap ctrl then i in quick succession as the computer booted.
Hope that helps.