I'm also using mine on my Pismo with Tiger: 10.4.6
I've noticed it likes one USB port more than the other, but the symptom is that the application can't find the scale and then quits, not that it can find the scale, but won't tare.
Sounds more like a scale issue than an application isse. Have you tried putting stuff on the scale and then hitting tare?
Don't ask who the bell's for, dude. It's you.