What version of Wildfire are you trying to use the plugin with? The version you linked to will only work with Messenger 2.3.x. If you need one that will work with 2.4.0 let me know.
A new version (1.3.0) of the registration plugin that has the ability to allow web-based registration is url=http://www.jivesoftware.org/wildfire/plugins.jspnow available[/url].