Well, the lead is attached to an antenna that is attached to the lid behind the display. Go to eBay and search for "tyco minipci" and you'll find a bunch for sale - they're cheap. You will have to follow the Dell service instructions for disassembling the lid and removing the display panel to get at it, but replacement is trivial once you get there.
As for a card, since you're running OS X you'll want a card that works there. The Dell 1505 is supposed to be good, as are Apple Airport cards. (The Intel 5300 I have does not support OS X.)
You may find that just replacing the antenna lead is enough - the card may not need anything. Those antenna connectors are delicate.