Problem: Asus Nexus 7 not recognized by adb when correctly installed and connected
Setup: Asus Nexus 7 tablet Windows 7 64 bit OS
Steps taken:
What worked for me is downloading the USB driver from the ASUS website:
http://support.asus.com/Download.aspx?SLanguage=en&m=Nexus+7&p=28&s=2
(you have to select the OS, which sometimes doesn't work, e.g. on the link Google returns when searching for "nexus 7 driver"). In that case, click on "Drivers and Download" and search for "Nexus 7". That gets me a page with a working OS selection.