Is the ethernet card a PCI card or one of the custom Comm Slot II cards? The CSII cards are tiny little stinkers, and should be easy to recognize. They don't look anything at all like PCI cards, so you can't miss it if you are looking at it.

The CSII cards are natively recognized by Mac OS, and they shouldn't need any special drivers. However, PCI cards do require special drivers (if they don't carry an Apple logo on them). Finding the appropriate driver may be problematic, though there is a good chance that the card is either an Asante, Dayna, or other well-known Mac ethernet card.

Peace,

Drew