ArgoUML is the leading open source UML modeling tool and includes support for all standard UML 1.4 diagrams. It runs on any Java platform and is available in ten languages.
ArgoUML 0.26 and 0.26.2 were downloaded over 80,000 times and are in use all over the world.
If you need to download, read the manual or write to the forum your own personal difficulties, you have to visit just one single webpage.
If you prefer a you tube video, it is here.