The installation of additional components/plugins/modules in Joomla 1.5 is quite easy.

All you have to do after downloading the extension 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 remeber to downlaod the AUser Manger package for a fresh install