You will need AirPort Utility....the application that you used to originally configure the AirPort Extreme.
Click Manual Setup
Move the focus of your eyes to the uppermost top of the computer's screen....(not the top of the window in the center of the screen).....to locate the AirPort Utility menu bar.
Click the Base Station menu, then click Equivalent Network Password
Your network "password" and also the long Hex "key" will be revealed to you.