If the double-click does not work fine, you can try the following command:
java -jar plantuml.jar -gui
You can then select a directory that will be periodically scanned by PlantUML. Alternatively you can specify the source directory as part of the command: java -jar plantuml.jar -gui /source/directory
The following window lists all images that are generated by PlantUML. If any modification is done in the directory, the window will be updated.