Hey guys I had the same problem. And i check it on three different machines all running different operating systems. Then I found ... arduino uno - Mac OSX Yosemite no serial ports showing for Uno R3 - Arduino Stack Exchange
Seems that even after i installed that USB driver that the first guy said, it still didn't work. Someone else suggested that I CHANGE THE ACTUAL WIRE.... seems that its just for charging not for data transfer and it worked. If you have another wire try to check the other wire. it worked for me.