For Windows: First: make sure you have current version of Arduino and note the location of the DRIVERS (very important). Second: check that you have selected the correct board type in Arduino IDE- Tools.Third: when you plug in the Arduino via USB, the "found new hardware" wizard should appear. There is an option for where to look for the driver. Direct the browser to the exact folder and you should be good to go.I spent a lot of wasted time because I thought I had directed the driver browser to the correct file when in fact I had directed it to a higher level folder. Also: try uninstalling the device from the device manager and then plug it in again to get the hardware wizard to appear.