The installation of additional components/plugins/modules in Joomla 1.5 is quite easy. All you have to do after downloading the component you wish to install is go to your Joomla admin area > Extensions > Install/Uninstall.
In the Upload Package File section click Browse and locate the archive of the component/module/plugin you wish to install. Then click Upload File & Install to upload the file and complete the installation.
Please note that components built for Joomla 1.0.x cannot be installed on Joomla 1.5 unless Legacy mode is enabled. More information on how to install Joomla 1.0.x components to Joomla 1.5 is available in this article.
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