ZED-R16 help w/mac

New to Firewire & ZED. Have firewire connected to mac and am recording in garage band. Records OK but can't seem to find how to control recording levels thru board rather than adjusting the preamp on each channel. Faders on the channel and the LR faders do nothing. Can I get the ZED channel fader to control it? Am locked out of the input level function in the edit window on Garage band. I have gone to preferences on the mac-selected midi/interfaces and chosen the ZED as my input source and specified what channel to assign to the GB instrument track. Any help w/b greatly appreciated.
Thanks!!

Recording levels are always set at the preamp gain no matter what kind of setup (from home studio to million dollar)... it's a good thing you can't do it anywhere else because such a thing would result in excess noise.

thanks for the help. I appreciate it!