Arduino Uno R3 doesn't show up in Mac OS X (10.6.8) system with USB?

Using Arduino IDE 1.x
8

I got the same issue, I try tons of thinghs, but the solucion was at bit of distance.
I just programm again the xxu2 (in my case the USB2SERIAL with 8u2 chip)

I leave here the link to use the dfu programmer (in mac)

https://github.com/arduino/Arduino/tree/master/hardware/arduino/firmwares/atmegaxxu2/arduino-usbserial