This count for Nexus 7 and may count for others as well.
Normally you go to settings / Applications / Development / USB debugging but from Android 4.2 the Development part is hidden.
You unhide it this way:
Go to Settings / About phone.
Find the Build number
now Tab on Build Number repeatedly about 7 times.
After touching 7 times you see a message 'You are now a developer' and the developer menu is back. It is found in Settings / System.
2016:
Now it is Huawei P8 lite with Android 6.0, and you still have to unhide the developer menu.
Go to settings / About the phone and tab on Build number (Versionnummer in Danish) 10 times. The developer menu is now found in Settings.
Best regards
Viggo Poulsen
Edited 2 time(s). Last edit at 11/21/2016 08:45PM by Viggo Poulsen.