Sorry your still having problems.
When you go to Apple Menu/System Prefs/Sound and click on the output tab does the HiFace show up as a selection? (you do have it plugged in correct? Just trying to cover all bases.) If it is not showing up try using a different USB cable (if you are using one). If you cannot get the HiFace to show up, the unit may be damaged.
Try what Pettyofficer suggested and try another USB port or try another USB device on that port. Are you using a USB port on the Mac or a USB hub? Some devices don't play well with the hubs.
To check if the driver is installed open your main HD/System/Library/Extensions and see if a file named Hiface.kext is there. If it is, the driver is in the correct place.
Also what OS are you using 10.4, 10.5 or 10.6?