USB on Mega and Nano

Hi, I have the Mega and the Nano.

I use Arduino IDE 1.6.5 on OS X E Capitan.
When I plug in the USB, the IDE recognizes the Mega right the way with /dev/cu.usbmodem..... No issue there. However, when I plug in the Nano, the IDE would not detect the Nano.
Why is that?

The Nano use the FTDI chip for USB so you need to install the FTDI driver.