Hi Marc,
If I understand correctly, with your new Sound SFX created, you went to the Contents Editor and added a Wwise MP3 Input source.
All you have to do now is double-click your Wwise MP3 Input row. This will prompt the Source Editor loaded with your Wwise MP3 Input. Also, if you have the Source Editor view already open, you can change the loaded source by selecting any other one listed in the Contents Editor.
We'll have to make that clearer in the doc!