For those of you (like me) who had used EquipCompare for a long time are aware that this addon hasn't been updated for Wow 5.0 and no longer works. There is a repalcement called MoncaiCompare and located at http://www.curse.com/addons/wow/moncaicompare .
You can also just hold down the <Shift> key in game. That was added a long time ago. I realize the addons made the feature "always on", but most people, including EC's author, thought the addons were overkill, and stopped using or updating them.
For those who want an addon, thank you for the link.