based on this, I'd say one of two things is the problem, likely the latter but possibly the former:
Antennas are broken somewhere other than the connector. It's possible, though unlikely. MacBooks (in fact most laptops) are designed in such a way to prevent the antennas/cables from taking damage during normal use. unless you have some significant damage to the chassis of the device, it's unlikely that this is the problem - unless of course, you DO have dings and dents in the chassis, specifically on the monitor-side of the notebook, in which case, this becomes MUCH more likely.
The alternative is that either your TX amplifier isn't engaging, or is electrically damaged in some way. In which case, you need to replace the WiFi card in your macbook to resolve the problem. You should be able to find one relatively inexpensively second-hand. I'm not sure if Apple has booby-trapped the WiFi cards at all, so you may want to do some research on this before proceeding (assuming you can replace it yourself at all - not sure if the card is a part of the mainboard or not).
in either case, the symptoms point to: you can hear them, but they can't hear you. Evidence is pretty simple, you can "see" the networks, and you haven't noted the signal strengths are all super low, so I assume they look normal; but when attempting to connect, it fails to permit the connection. likely because the target network isn't able to receive your signal.
This is all just assumption based on the evidence provided, and there are some crazy other things that could have also gone wrong, but IMO, are much less likely than the issues I've stated. Since a new Airport card can be had (from eBay) for $20-30, it's not unreasonable to pick one up to test with. Alternatively, buy a USB wifi dongle and use that until you can put together something more permanent.
Personally, I'm not a fan of USB WiFi, but if it's your only option, it's better than nothing. but I'm not you, maybe you're perfectly happy with USB wifi.
