Had to help a couple of folks out with this last week. First time I used apples tutorial, but this didn’t work. The second time I used Rackspace’s instructions and it worked. The difference was using full email address in the user field. Below is a link that got it working for me.

http://www.rackspace.com/knowledge_center/article/set-up-exchange-2013-on-ios-devices

or

http://www.rackspace.com/knowledge_center/article/setting-up-your-iphone-ipad-ipod-with-microsoft-exchange-2010-e-mail

Make sure your Wifi is turned off.