At the moment, Help > Help gives me the message:
The documentation isn't installed.
The ".tar.gz" or ".7z" archives are available here:http://www.bouml.fr/documentation.html
Extract an archive and set the environment through the Miscellaneous menu to indicate where the directory "doc" is then close this dialog and redo
This isn't very helpful! I think the modern way would be to just launch the default browser with a help documentation URL at bouml.fr. To make help available offline, maybe you could provide a button 'download and install help' on the tool, which would do a http get to obtain a .zip, but I think realistically this isn't that important - being able to get to the online help is!