Swipe down from the top right of the tablet screen => Tap Settings => Tap WiFi => Long press your connection SSID => Tap Revove => Setup the WiFi connection again. You will now be allowed to enter your key.
Note that I have encountered multiple instances when the Nexus 7 would fail to connect to routers that use WEP encryption 64 (10 numbers). Simply changing the encryption in the router to WPA or WPA2 corrected the issue.
Sent from my Nexus 7 using Tapatalk