Go under the Apple in the upper left hand corner to "about this Mac" > more info > Network > Airport and see if your card is being seen. If not reseat the card, if it still fails, replace the card. Here's how to do it: MacBook Core 2 Duo AirPort Card Replacement
According to which machine you have, here are the cards: Relevant parts
MacBook Core 2 Duo Santa Rosa/Penryn 802.11n Airport Extreme Card
MacBook Core 2 Duo Santa Rosa/Penryn 802.11n Airport Extreme Card
MacBook Core 2 Duo 802.11n Airport Extreme Card
MacBook Core 2 Duo 802.11n Airport Extreme Card