First, you should focus only on Sequence Diagrams. Even if you are not interested in sequence diagrams, since they are working without Graphviz, it is somehow simplier to let them work. And if they do not work, other diagrams will probably also not work.
You should then test Sequence Diagrams using the command line, and not in the target tool you want to install (Eclipse, Word, Mediawiki...) :
In the sequenceDiagram.txt file, you can have a very simple test:
Alice -> Bob: test
The -verbose flag is used to generated some logs that you can send to us, if no image is generated.
Under Unix, you must have X11 librairies installed (which is done by default). Otherwise, you will have some HeadlessException.
You may have to add a flag -Djava.awt.headless=true in the command line.
If this is working, but not in your tool, it is probably due to a specific tool issue:
On MediaWiki & Windows, you must set the read & execute permissions on C:\Windows\system32\cmd.exe for the account the PHP web service extension is running under. PHP needs those permissions to "fork" other processes.
Once sequence diagrams are working fine, you can focus on other diagrams.
If Graphviz is not installed on your system, you have to install it first.
Under Linux, try to launch dot command into a command shell. You may have to finalize the installation by typing dot -C.
You may have to set GRAPHVIZ_DOT variables if you have not chosen the default installation directory of Graphviz.
To test the installation of Graphviz, you can use the command line:
java -jar plantuml.jar -testdot
You can also use this special diagram description:
Once again, you should test using the command line and the -verbose flag: