Once you downloaded the SweetHome3D-3.6-macosx.dmg file, search it in the Finder (it should be in your Downloads folder) and double click on the SweetHome3D-3.6-macosx.dmg file. This will open a window where you'll see many license text files and the Sweet Home 3D application represented with the following icon:

Drag and drop this icon to your desktop or to the Applications folder of your system to copy the application. That's all!