Networking problems, no matter what Mac you're having them with, go in the Networking forum, NOT the forum for your computer.
And chipchen's advice is on the mark. Antenna cables seem to come out of AirPort Express cards pretty often. Look up the instructions in Apple's Knowledge Base for installing the card in your particular model of computer-that will tell you exactly how to get at it. Go through the antenna cable installation steps to ensure the cables are firmly installed, then maybe put a piece of celophane tape over the connector and onto the card to secure it. Put it all back together and odds are you've fixed your problem.