Download the disk image from http://sourceforge.net/projects/qse/files/. You want the "qse-x86-x.y.z.dmg" or the "qse-x64-x.y.x.dmg" file.
When you have downloaded the disk image double-click it and the disk image will be mounted as if it were an external disk. A finder window will open up containing an application. Drag this application to wherever you woluld like it to be installed. Double click it run the application.