Alesis USB adaptor knocks out my monitoring.

Hi,

I've bought an Alesis Jack to USB adaptor to plug an electric guitae into my mac and I am using Garageband.

As soon as I plug the USB lead in my output dissapears. I can hear other stuff- playing stuff in Safari etc... But nothing from Garageband. I can see the levels are there but I can hear nothing.

Anybody got any idea about this?...

Thanks for any help you might have.

R.