I have bought 10 Arduino Mega 2560 clones and all of them have the same problem. When I connect the to my PC I get a message that windows cannot recognize that device and the LED at the board in R17 blinks non stop. normally this led stops when the arduino is ready. Also in the Arduino IDE I don't see the port option enable.
You need CH340G drivers. CH340G is a chip that basically replaces the usual ATMega 8U2/16U2 that is used for USB to serial interface. As CH340 cannot be reprogrammed it means that you can't emulate other USB devices with your arduino for example a keyboard.
You can download the drivers here.