When I try to run from XCode 6.4 I get "iPad may be running a version of iOS that is not supported by this version of Xcode." It won't let me open the Xcode beta either, saying it needs to be on OS 10.11. I need to install an app right now though. Is there a way to do it from Yosemite?
Apple have recently dropped support for Xcode on Yosemite. The last supported version is 7.2.1. If you want to use Xcode 7.3 which supports iOS 9.3 and above you'll need to use El Capitan which is a free update for Yosemite users.