The preferred way to uninstall programs in Windows is to use Add/Remove Programs option in the control panel, which is accessible from the Start button on the lower left corner of your computer desktop. A dialog box (usually named Add/Remove programs) will display a list of programs. To remove MEGA, scroll down to MEGA so that it is highlighted, then click Add/Remove.
The preferred way to uninstall in Mac OSX is to simply drag the program from your applications folder to the trash bin.