Volume keys in combination with external soundcard; How?

Hi folks,

I recently bought a new MBP alongside a Focusrite Saffire 6 USB external soundcard, and an Apple wireless keyboard. As of now, if i want to adjust the volume of the music I'm playing I have to turn the knob on my Focusrite Soundcard, since using the volume controls on my keyboard show up as disabled; when I press them the volume logo shows up on my screen, but it denies me to alter the volume.

What's the way to go to enable this? I already checked the Audio & Midi preferences, to no avail.