Sounds like you guys have missed the part where the Flex API is no longer included in the Flex Viewers source code due to the new Apache licensing. you need to download the API separately and copy it into the libs folder for your project.
@Andrea,
Are you trying to access the application builder, or an application created with the application builder?
The application builder is an AIR application (not a web application, but more of a "desktop" application) and thus does not run in your web browser. It is accessed from the Start menu (or its non-English equivalent). You should not be copying AppBuilder.swf anywhere...
I just installed Adobe Air, dowloaded and unzipped the AppBuilder-2.5.zip file and tried to run AppBuilder.swf.