It's right above the payment stuff...there should be a Redemption options. It may not show up if you have a non-qualifying itin (e.g., codeshare).

While many say the CO site is the "best," I, too, miss obvious things sometimes because it's so cluttered. I had trouble finding where they buried the confirmation number on a reward booking the other day that I had to use my browser's Find feature to find it hidden in the middle of the page, past a bunch of ads for stuff I don't want. You would think that the confirmation number is one of the most important items to display after clicking purchase.