To Unlock ADB Debugging also known as USB Debugging for your Kindle Fire HDX you just have to follow these steps.
Go into your Kindle Fire HDX Settings
Click on Device Options
Then you will want to tap 7 times on Serial Number, (Yes, this doesn’t look like it does anything) but once you start tapping a few times, it will say 5 more taps until Developer unlocked, 4 more taps until developer unlocked…etc.
Once you reach your 7th tap on your Kindle Fire HDX Serial Number you will see Developer Options open up at the bottom, if you continue to tap it will say No need, you are already a developer.
In Developer options make sure Enable ADB is On, but it looks like it turns on the moment you unlock developer options.
You may now happily run debug functionality on your Kindle Fire HDX! Why am I doing this you ask?
Well, there is an application called Wondershare MobileGo which allows you to manage your Android device including the Kindle Fire HDX and I plan on doing some pretty neat things with it. More to follow in the near future, stay tuned!