Unplug the device from your Mac & power down the device completely (hold the power button for several seconds; slide to power off).
Close Xcode and Instruments.
Restart the device & once it has booted completely re-connect it to your Mac.
Re-launch Xcode. Here, my device showed as disabled and Xcode indicated that the device was not available for use.
Open your project; clean (Shift+Command+K), Build (Command+B), Profile (Command+I).
After Instruments launched I noticed that the device was enabled. Upon selecting it, a message was displayed with the title
Enable this device for development?and message
This will open Xcode and enable this device for development.(Note that this only happened to me the first time I went through this process even though I had already been using the device for development - whereas some users have also reported that they are not presented with this dialogue.) Enable this device for development?
Open Xcode. Here Xcode did not prompt me for anything nor was anything displayed - no additional messages indicating anything had been done or that the device was or was not available for development. Opening the Devices window, the device appeared to be available. (I have not been presented with this option for subsequent occurrences.)
Now I was able to select the device in Instruments and profile it.