I could not find mention of Mac compatibility ANYWHERE about this NIC. Sorry, the device will most likely not work.
You need to return to the store where you bought it--NIC and receipt in hand--and ask to speak to a manager immediately. In a calm, firm voice, let that person know about the shoddy advice you received, who gave you the advice, and then wait to see the manager's reaction.
He should apologize immediately, refund your money, then apologize some more. You should be gracious and accept the apologies.
NOTE: For everone tuning in, this is a good object lesson on how WE ALL (as consumers) should treat people in stores. I'm horrified at some of the audacious (and childish) behavior posted in some AT threads (especially in "Hot Deals," of which I am a regular). Respect goes both ways, and if you expect to be treated with respect, you should also demonstrate you are capable of reciprocating. I''m not making excuses for idiot store managers, clerks, or salepeople (especially the numbnuts originally mentioned in this thread), but go in with a chip on your shoulder and you should fully expect to meet someone else with a chip on their shoulder.
All that being said, I have been and continue to be completely APPALLED at some of the patently WRONG (at best) or insidiously LYING (at worst) information I have overhead sales associates telling customers in stores.
Finally Mark, here is a tidbit that will kill two birds with one stone...the mnemonic I use to remember the OSI Model of Networking:
Layer 1 = Please = Physical
Layer 2 = Do = Datalink
Layer 3 = Not = Network
Layer 4 = Take = Transport
Layer 5 = Sales = Session
Layer 6 = Persons' = Presentation
Layer 7 = Advice = Application