You probably need to unlock your Network settings. Look for the lock icon/graphic in the lower left corner of Network settings window -you'll see the text "Click the lock to make changes" next to the icon.
Thanks. It worked for me, finally! I'm using OS X 10.7.4. I added the 18.104.22.168 to the DNS list. Clicked OK which closed the DNS window. Clicked APPLY on the Network window with WIFI selected. Had to Restart both my iMac and Facetime to get the sign in to finally work.