If you connect your Android Galaxy Nexus or any other Android phone to your OS X computer and the Android File Transfer app is not opening automatically or even if you open it manually, you get a message saying that there is no device connected. You simply need to turn off USB debugging.
While your Android phone is in USB debugging mode (this may not apply to all the phones but it certainly applies to Galaxy Nexus), android file transfer does not recognize it. Once you turn off the USB debugging mode, android file transfer should start recognizing the device once again.