Make sure you have v0.0.0.10 of the Chrome app, you can check in chrome://extensions - if you have an older version the easier way to update is to restart your Chromebook
Once updated, plug in the Nano and click "Board at /dev/tty" or "Select Other Board & Port" from the board selection dropdown in the Create Web Editor
Select the "Arduino Nano" (you'll have to scroll), and the desired port.