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.

More infomation on this topic can be found at : http://help.joomla.org/index2.php?option=com_content&task=findkey&tmpl=component;1&keyref=screen.installer.15