Hi, welcome to the PlantUML plugin for MediaWiki. The full project page for this plugin can be found on

http://www.mediawiki.org/wiki/Extension:PlantUML

Prerequisites

A Java installation. Can also be OpenJDK
GraphViz extension
Prerequistes of GraphViz. Basically if you get the GraphViz extension running you should also be able to get PlantUML to run.

Installation

With Composer

[Composer support is in preparation] (https://www.mediawiki.org/wiki/User_talk:Legoktm#Adding_Extension_PlantUML_to_packagist)

Without Composer

Go to the extensions folder of your MediaWiki installation. On RedHat and derivates this will be /usr/share/mediawiki/extensions. cd /usr/share/mediawiki/extensions
Create a new subdirectory PlantUML in this folder and move to this directory. mkdir PlantUML && cd PlantUML
Move the PlantUML.php file into the PlantUML directory. mv <downloaddir>/PlantUML.php.
Choose your usage style. You can either process images locally (on the server where MediaWiki was installed) or in the cloud. The local version supports SVG-images and embedded urls, at the cost of local processing. The cloud version is lightweight for your server, but does not support embedded urls (yet) and is (still) stuck with PNG-images. Default local processing is expected. If you want to use the cloud, please edit the PlantUML.php file and change $usecloud to true.
When using local processing: get the plantuml.jar from SourceForge

wget https://downloads.sourceforge.net/project/plantuml/plantuml.jar
(Optional) Edit PlantUML.php and change the variable $plantumlImagetype to your preference. Mind that SVG produces the superior graphics, but that only PNG-images and image maps are "rock solid". If you use the cloud, it will always generate PNG images. Local processing defaults to SVG.
(Optional) Adapt the getUploadPath and getUploadDirectory to your preference if you want these different from MediaWiki's standard settings. Mind that these directories must be writeable by the system user who runs MediaWiki.
Put the following line near the end of your LocalSettings.php in MediaWiki's root folder to include the extension:

require_once('extensions/PlantUML/PlantUML.php');
When using the cloud, make sure that httpd can submit HTTP-requests.
Reload http `service httpd graceful
Enjoy!