Arduino Due doesn't recognized by win 7

I installed IDE 1.5.6 r2 and connected my Arduino by a micro usb to usb port but my laptop doesn't recognize arduino as a device. I tested the arduino on my MAC laptop. the same. so I returned it to the store they were able to program it by their computer but they told me there is a problem with programming usb and they changed the arduino.

the new Arduino has the same problem. I'm using this cable

not sure if it is the source of my problem. L led is blinking while arduino is connected to the computer.

I am a beginner and have no experience with arduino. it is very frustrating that after 2 weeks I could not run my arduino.

Thank you for your help in advance.