The ways to enable USB Debugging mode vary from one Android version to another. USB Debugging is required by adb to be able to connect to the device. It's an important step to make sure communication will be established between the computer and your android device
Android V2.0 To V2.3.X
Settings > Applications > Development > USB Debugging.
Android V3.0 To V4.1.X
Settings > Developer Options > USB Debugging
Android V4.2.X To V4.4.4
Settings > About Phone > Build Number
Android V5.0.X Lollipop
Settings > About Phone > Build Number