I bought an Arduino Uno R3 a few weeks ago and have been playing with it successfully running IDE 1.6.2 on OS X 10.10.2. Today I was looking into one of Adafruits libraries that said it had some issues with the 1.6 IDEs so i downloaded the 1.0.6 IDE.
After opening up the 1.0.6 IDE, I could no longer see the serial connection through the Tools -> Serial Port menu.
The only other thing I changed was i used an external power adapter for the first time (12v 1.5A).
I don't see how either of these would affect the serial connection but it just will not work.
Run "System Information" and check that your board still shows up under USB. If it doesn't there is something wrong with the Arduino, the cable, the USB port, or the USB drivers. Have you tried going back to USB power? Did you have anything else connected to the Arduino? Is it a genuine Arduino using an ATmega16U4 for the USB to Serial or a clone using some other chip?