If you just updated an Android device to 4.2, and try to develop for it, you may have noticed that there are no Developer Options anymore. They used to be in the settings, just above "About phone", but since 4.2 they are gone. This can make it difficult (impossible) to enable USB debugging. However there is a fix for it that once you know it is simple enough.
All you have to do to enable it is to go to Settings->About phone. Then go down to Build number, and tap it 7 times. After the first three times there will be a count down towards the hidden option. Just go back to the settings menu, and the Developer options will have appeared.
I found this from this forum thread.