I tried to debug my WatchKit app on a real Apple Watch today. After hitting the Debug button in Xcode, the main iPhone app was installed properly, but the Apple Watch only d
Basically you need to perform two steps:
XCode can manage the whole process for you:
1. Deploy the App to your iPhone/iPad
2. Start the WatchKit App also on your iPhone/iPad (not in the simulator)
3. Choose Fix Issue, when the code signing error message appears
4. Select your Development Team and let XCode do the rest
I've also written a short tutorial for this.