If you want to use it on your local mac desktop, you can use Homebrew to install that. Before installing PlantUML, make sure that you have installed Java and Graphviz.
Make sure that you have installed Java:
$ java -version
$ brew install graphviz
Now, install PlantUML:
$ brew install plantuml
Let's try a sample.
$ wget https://gist.githubusercontent.com/QuantumGhost/0955a45383a0b6c0bc24f9654b3cb561/raw/08e6eb965a5604a8ba89a586707708a9a9b98c1a/example.puml
The below command will dump to png file
$ plantuml example.puml
The below command will diaplay the latest result
$ plantuml -gui example.puml
As a side note, you can integrate it with Visual Studio code or sublime editor. If you are using Visual Studio code, I would recommend PlantUML extension which is able to show a live preview.
A sample for planting sequence diagram: