After launching AU Lab, press the "Create Document" button in the dialog that appears. A mixer graphical user interface will appear, with an area titled "Effects". Here you can select Soundness>SoundSoap and enter your activation information.
If you are using Sound Studio, which does not allow activation because of "sandboxing", you will want to use AU Lab or some other plug-in host application to activate SoundSoap 3 first. A similar issue exists with Garageband X.