Solution To Nexus 7 Doesn't Show Up As Media Device (MTP)
1. Plug the Nexus 7 into your computer with the USB cable.
2. Open Device Manager.
3. Find "Android Device" near the top and expand the node. Double click on "Android Composite ADB Interface".
4. In the properties window that pops up, uninstall the driver (chose to delete the current driver as well).
5. Unplug the tablet and plug it back in.