Make sure it is enabled it in the extension manager. Customize (on the menu bar) > Extensions. Select the extensions you want to enable and then close it. That toolbox should already be visible in "toolboxes > system toolboxes" in the catalog window (maybe only ArcGIS 10). In 9.3 you may have to then turn the toolbar on. Customize > Toolbars > 'Locate the toolbar'. Then i believe you can dock it or just let it float. From there you should have access to all of the tools within that extensions toolbox.
If when trying to enable the extension (customize > extensions) you get a prompt that "There is no [extension] license currently available" then you may have installed it wrong.