Next time, please add a link to your product.
That clone was cheap ! wasn't it ? If you paid more than 14 dollars, you paid to much.
Search on Ebay for : ch340g mega
It uses the CH340 or CH340G or CH341 chip for the usb-serial.
In linux you don't have to do anything, it works right away.
For Windows 7, you need a driver from http://www.wch-ic.com
The zip file or the exe file.
http://www.wch-ic.com/download/list.asp?id=127
For the mac, you have bad luck.
Perhaps this driver is compatible with the CH340: http://casbi.nl/
The official Arduino Mega 2560 needs drivers that are included in the download of the Arduino software.
The CH340G chip is no where near the official Arduino usb-serial chips. I have seen them only in the last few weeks and bought a board with it myself.