With the release of Joomla 1.5 back in 2008, the old plugins, themes, modules and components which were designed for Joomla 1.0 might not work properly. This is because of the fact that the core is completely rewritten in Joomla 1.5. As a result, lot of existing extensions, plugins, templates and modules for Joomla version 1.0.x does not work or fail to work properly in the Joomla! 1.5 and this might cause error. But you can use these plugins using the legacy mode which is made available in Joomla1.5.
To enable legacy mode in Joomla 1.5, login to Joomla administrator dashboard (www.yourdomain.com/administrator) and mouse hover on the Extensions menu and click on Plugin Manager.
This will open the plugin manager, search for Legacy using the filter available and you can see plugin called “System-Legacy”. Now enable this plugin either using enable button or cross mark on the side of the plugin.
Once enabled, you can see legacy mode in action on the dashboard.
Now you will be able to use the plugins and themes created for Joomla 1.0.