Not sure if you figured it out yet, but you have to get the driver from SDK Manager (comes with the Android SDK). In SDK Manager it's near the bottom of the list under "Extras" and is simply called "Google USB Driver". You also have to enable USB debugging on your Nexus 7. I don't have that device, but on mine it's in the Settings menu under Developer Options titled "Android debugging".
My post is very late and there's a good chance you've figured it out by now, but maybe somebody else will find it useful.