Nexus 7 not visible over USB via "adb devices" from Windows 7 x64

In addition to setting USB connection/storage mode to "Camera (PTP)", I also had to enable developer mode, which has been hidden since 4.2.

1. Go to the Settings menu, and scroll down to 'About phone/tablet'. Tap it.
2. Scroll down to the bottom again, where you see 'Build number'.
3. Tap it seven (7) times. After the third tap, you'll see a playful dialog that says you're four taps away from being a developer. Keep on tapping and you've got the developer settings.
4. Now under Settings there's 'Developer options'. Tap it.
5. Tap 'USB debugging' to enable it.

Source: How to enable developer settings on Android 4.2

I discovered that you can also do Device Manager -> Update Driver Software -> Browse my computer for driver software -> Let me pick from a list of device drivers on my computer -> Android Phone -> [ADB driver version 6 near top of list... sorry, I can't remember exact name]

As soon as I did that, it connected, and I was able to sideload version 4.2 on Windows 7 64 bit.

To fix/install Android USB driver on Windows 7/8 32bit/64bit:

1. Connect your Android-powered device to your computer's USB port.
2. Right-click on Computer from your desktop or Windows Explorer, and select Manage.
3. Select Devices in the left pane.
4. Locate and expand Other device in the right pane.
5. Right-click the device name (Nexus 7 / Nexus 5 / Nexus 4) and select Update Driver Software. This will launch the Hardware Update Wizard.
6. Select Browse my computer for driver software and click Next.
7. Click Browse and locate the USB driver folder. (The Google USB Driver is located in <sdk>\extras\google\usb_driver\.)
8. Click Next to install the driver.

If it still doesn't work try changing from MTP to PTP.

When the Nexus 7 is plugged in there is a persistent notification that indicates "CONNECT AS / Media Device (MTP)". In this state adb devices will not show the Nexus, or undoubtedly any other device. Not exactly obvious, but if you select the second option "Camera (PTP)" the device is available for debugging (the lesson is ignore the camera, and focus on the protocol PTP).

This configuration is persistent, and I'm guessing that with a brand new device it will connect as MTP until told otherwise.

Thanks to @Ciaran Gallagher Settings --> Storage --> Top Left Option (Computer USB Connection) tap--> choose MTP