Topic: Ficeface 800 and Total Mix Physical Volume Knob

Hello
I just purchased a Fireface 800 Sound Card  and I installed the latest drivers on my Imac to find out that there is no Physical volume knob on the card, so I purchased a Griffin Thechnology Power Mate USB (Midi) device to use as a volume knob for the Fireface 800 but it doesn't seem to work. Can someone  please guide me to make this setup happen?
I really wish to have a physical volume knob and this Griffin unit cost me 60$ and I would love to configue it to work as a main volume knob in TotalMix.
Thank You
Regards
chrisj747