Soundflower not working

I just downloaded Soundflower 1.5.2 and installed it on my MacBook Pro running 10.6.8.

In the Sound prefs. pane I selected 'Soundflower (2ch)' as the output. I expected this would kill the internal speakers but I still hear sound from them (regardless of whether 'Built-in output' is checked in Soundflowerbed).

While playing a track in iTunes, I opened Digital Performer and recorded to a track with 'Soundflower (2ch)' as the input, but there's no signal.

Any idea what I'm missing here?

Thanks in advance,
Steve = : ^ )