JOHNKOVO  Make sure Airport Express is connected to your stereo (my Airport Express is hooked into Aux. on my amp), the green light is on and recognized by Airport Utility on your Mac.

Quit iTunes. Activate Pandora. Activate Airfoil and select Firefox (or whatever browser you use to listen to Pandora) in the drop down menu... then click the "volume" icon to the left of your named Airport Express (should be below your Computer icon).

Once you've clicked the volume icon, it should immediately turn from "Available" to "Active". Now you should be able to listen to Pandora through your external source.  
(Version 3.3.2)

JOHNKOVO  Airfoil works!

I needed something that would bypass iTunes and stream Pandora to Airport Express. A quick search found this page and these reviews (thanks to THE_BARON whose situation was nearly identical to my own).

5 minutes after downloading Airfoil I was listening to Pandora through my living room stereo and could not be happier.

This app is amazingly simple to set up and use... absolutely no learning curve... just click the speaker icon and listen. If only all programs were this easy.

Airfoil is a brilliant solution for anyone who is looking to stream web radio to Airport Express.... period.  
(Version 3.3.1)