Hello Icelander,
Yes, all of the VI Labs products can also be used on Mac but the UVI Workstation requires Mac OS X 10.7 or higher to run.
The first step is to make sure the Mac OS X CoreMIDI driver is selected in the "MIDI Drivers" dialog, as opposed to Apple DLS Synth. Then from the "Select Ports/Instruments" dialog you can select "BIAB Virtual Output" for the instrument you would like to send to the UVI Workstation.
Next, open the UVI Workstation in stand-alone mode and select the BIAB Virtual Output as your MIDI input. Now you will be able to hear the piano playing along with your audio or MIDI tracks in Band-in-a-Box.
If you wish to record the sounds coming through the UVI Workstation, either alone or with the Band-in-a-Box mix, you can use Soundflower. Soundflower is free and is available
here.