Please:
- go to Settings, and make sure you have "Developer options" at the bottom (immediately above "About tablet")
- click on "Developer options", and check "USB Debugging"
If you do not see "Developer options", click on "About tablet", and then click on "Build number" until you get "Developer options"
The first time you connect your PC to the Android device, make sure you authorize your PC.
After this, adb should work.