where do I find my Airport Express password?

If you have a Mac that can connect to your wireless network and you're using an Apple AirPort Base Station, then simply use AirPort Utility. From the Base Station menu, select Show Passwords...

If you don't have a Mac, reset your wireless network password by momentarily depressing the reset button on your AirPort Extreme, Express, or Time Capsule. Press the button for one full second, no more. Then, configure a new password using AirPort Utility.

If you can't connect to your wireless network, or you are not using an Apple AirPort Base Station, you can use the procedure that follows below to retrieve it from your Mac's Keychain.

Your wireless network password is stored in your Mac's Keychain.

You must first have an Administrator account on a Mac that you use with that AirPort Base Station, one that is used to connect to your AirPort network.

On that Mac, open the Utilities folder. Find Keychain Access and launch it.

In the search field at the upper right, type Airport. All the Keychain entries for your base station will appear there, as well as other entries.

Look at the column "Kind". The entry you want to select will be labeled "AirPort network password" (not "AirPort base station password" or anything else).

If you are trying to recover the AirPort Base Station password then the entry you want to select will be labeled "AirPort base station password" not "AirPort network password".

Double-click that, and in the window that appears check the box for "Show password".

You will get the following window:

In the Password field type your Mac's Administrator password - the same one you used to log in to that Mac.

Then, click Allow, and your wireless network password will appear in the "Show password" field:

Make note of it.

Close the window, then quit Keychain Access.