For any others who haven't been able to solve this one yet: if, like me, you decided to go with the cheaper chinese kick off arduino clone boards and had problems similar to this one, please check the following forum post, as it solved the problem for me:
And another blog site that explains the whole process really simply for noobs like me:
Actually the dude has even mirrored the relevant driver that solves the problem without having to find it on an all chinese or all russian website!
Yes, I have been struggling for a month or so, and finally realised (after reading http://forum.arduino.cc/index.php?topic=198539.15#msg1720630) that other chips were being used, in lieu of the FTDI chip, for example the Prolific 2305. After inspecting the board with a magnifying glass, I saw that I did not have an FTDI chip in my Chinese clones of an Uno and a Mega, but a CH340G instead.
Once I realised that I found the http://wch.cn/product/CH340.html site, but the Mac driver doesn't appear to be there. After a bit more googling I found the kiguino site. I downloaded the mirrored driver, rebooted the mac, and when I connected the Uno, my MacBook died and restarted with the grey serious error screen and refused to boot. After disconnecting the Uno, the Mac rebooted as normal, so no lasting damage done, but I am a bit wary of trying to plug the clone in again.
Anyway, note that the chip says CH340 on it, but the drivers are CH341. I assume that they are compatible.