Download the latest version of Structorizer (Mac) from http://structorizer.fisch.lu and unzip it somewhere on your hard drive. There is no special installation package. It is just a bundled app. Simply drag the "Structorizer" application to your Application folder or run it from where you have unpacked the archive.

When you run Structorizer the first time, it will register the file types ".nsd", ".arr", and ".arrz" and associate them to itself, so you can doubleclick NSD files (.nsd) and NSD arrangement files (.arr, .arrz) later on to open them automatically within Structorizer.