It's easy to take your question with two different approachs - verifying the hardware is connected correctly and then making sure the audio driver is selected in the software.
With Firewire interfaces, the best way to verify that your device is connected would be to look in your System information file. This helps you recognize if the device is connected and functioning (i.e. firewire card/cable/mixer is communicating) and also helps you recognize when the driver is mounted and seen by your computer.
You can access System Information by going in a New Finder window to: Applications >> Utilities >> System Information.
Any firewire devices seen by the computer will be listed under "Hardware >> Firewire" as shown here.
If you do not see your device listed in your System information file, you might first want to change your firewire cable out or make sure you have a Bilingual Firewire cable supported for proper connection to firewire 400 devices, If the device shows up it may require a Uninstall and Reinstall of the device driver just to make sure there are no remnants of older drivers still there (which is very common).
If you are still having issues at this point, then you may want to Contact Technical Support for further assistance.