This depends on the nature of the bug:
Problems with the core Rodin platform, as well as feature requests, should be filed via the SourceForge bug tracker: http://wiki.event-b.org/index.php/Bugs_and_Feature_Requests
To file problems with individual plugins, check the plugin’s documentation in the wiki (1.1.2).
If you are unsure whether to file a bug or not, consider asking a question on the Rodin user list at rodin-b-sharp-user@lists.sourceforge.net.
To report a problem with the handbook, use the feedback button that is present in the HTML and Eclipse Help version of the handbook.