Looking for About Phone in Settings. And scroll down till you see Build number. Tap here till you see Toast message tell you have just enable developer mode.
Back to settings, you can see options: "Developer options"
Open up your device’s “Settings”. This can be done by pressing the Menu button while on your home screen and tapping settings icon then scroll down to developer options and tap it then you will see on the top right a on off switch select on and then tap ok, thats it you all done.
In case you enabled debugging mode on your phone and adb devices is not listing your device, it seems there is a problem with phone driver. You might not install the usb-driver of your phone or driver might be installed with problems (in windows check in system --> device manager).
In my case, I had the same problem with My HTC android usb device which installing drivers again, fixed my problems.
Step 1 : Go to Settings >> About Phone >> scroll to the bottom >> tap Build number seven times; this message will appear “You are now 3 steps away from being a developer.”
Step 2 : Now go to Settings >> Developer Options >> Check USB Debugging
this is great article will help you to enable this mode on your phone
Navigate to Settings > About Phone > scroll to the bottom > tap Build number seven (7) times. You'll get a short pop-up in the lower area of your display saying that you're now a developer. 2. Go back and now access the Developer options menu, check 'USB debugging' and click OK on the prompt. This Guide Might Help You : How to Enable USB Debugging in Android Phones
Source: Stackoverflow.com