Enable "Developer" inside the "Settings" app on iOS Before Xcode 6 one could go to the Xcode Organizer window. Tap on the Devices tab. Select the connected device and click the "Use for Development" button. See here.

However with Xcode 6 there are no longer a "Use for Development" button. So how does one enable it?

Update

My Devices window looks like this.
The devices tab is now a new screen. Use for Development no longer exists.

The option in Window → Devices or ⇧⌘2. 

From that window you could enable devices for development by adding your provisioning profiles. Right click on device and select Show Provisioning Profiles.... It will appear a window where you can add your profiles.

After that you will be able to install your apps in that device.