Add brush isn't painting in weight paint mode.

All of a sudden it started working…

I’m doing a tutorial here: http://www.youtube.com/watch?v=HFVdHePUIfo I’m supposed to make the rubber connection from the chest to the upper arm have gradually lesser weight using the Add paint brush. But I can’t get the add brush to work at all. Can someone tell me what I am doing wrong?

From the tutorial:

What I get:

Blend file:
http://www.pasteall.org/blend/23994