The PlantUML Confluence Plugin depends on the Graphviz package. Please ensure that you have installed the package on your Confluence system.
The Plugin requires JRE 1.6.x or higher (see also Supported+Platforms).
After the installation of the plugin you should check if the plugin was successfully initialized by adding the PlantUML macro with the body "version". Have a look at Note to Users of Confluence 3 and 4 if you are running an older Confluence version.
This should result in a page that looks similar to the following output:
Note to Users of Confluence 3 and 4
The PlantUML Confluence Plugin works with Confuence 3.1 and newer.
However, version 4 of the plugin adds some modules that require Confluence 4 (xhtml support). Version 5 of the plugin adds some modules that require Confluence 5 (blueprints).
You will notice that when you install or update the plugin. You will likely get an error message like This add-on has one or more modules with missing dependencies. If the plugin is not enabled after installation/update. try to enable it manually.
The following screenshots show the correct installation of the PlantUML Plugin 5.x in Confluence 3 and 4.
PlantUML Plugin installed in Confluence 3
PlantUML Plugin installed in Confluence 4