This was something that was driving me up the wall and google wasn’t very helpful. For people who are stuck like me.
Install the drivers for your tablet used ADBDriver software (makes it easy) http://adbdriver.com/
Or install it manually http://developer.android.com/sdk/win-usb.html
Once that is done and it doesn’t work here is how to turn it on in L:
Go to settings -> about tablet and click on kernel version until dev options are enabled
Go to setings – > dev options and turn ON USB debugging
Go to settings -> Storage -> Hit the three dots in to top right -> USB Computer Connection -> and check MTP box.
It should now show up in my computer. IDK why they hid it away and made it a pain to find while being disabled by default.