Steve,
Are you using the compiled code or the app builder, if so you will not have that file as it is source code.
If you are using the viewer source code the file is an mxml file which is in the API download that does not come as standard with the viewer code, you have to download the API separately and add it to the viewer code.
https://developers.arcgis.com/flex/
Regards
Anthony
I am using Flash Builder and exported release build.
esri|PopUpRenderer { skinClass: ClassReference("com.esri.viewer.mycustomfolder.CustomPopup"); }
Make sure your PopUpRendererSkin.mxml file is in this exact location:
src\com\esri\ags\skins
If the PopUpRendererSkin.mxml file is in another location, then you would need to declare the new location of your file in the defaults.css like this (if I remember right):esri|PopUpRenderer { skinClass: ClassReference("com.esri.viewer.mycustomfolder.CustomPopup"); }