This solution is working for many people (including myself).
Make sure you actually do add your device [I actually hadn't] (log into apple developers -> members center -> certificates, identifiers, and profiles -> devices -> + button)
Download an old version of iOS (I used this link: http://www.mashinglab.com/download-ios-6-1-4-ipsw-file-for-iphone-5-ipad-ipod-touch-direct-links/)
Put your phone into DFU Mode (http://theiphonewiki.com/wiki/DFU_Mode)
In iTunes, hold down the option button and click restore, and browse for the file you downloaded in step 2.
Wait for it to restore to older version.
In iTunes, hold down the option button and click update (not restore!), and browse for the file you downloaded from the apple developer page for iOS 7.