Move the plug-in file to the plugins folder, i.e. your-xoops-root/modules/rss/plugins.
Point your browser to your-xoops-url/modules/rss/admin/index.php?op=plugins and you can tell if the plug-in is ready to be installed. Check the "install" box then click "Submit Changes" to install the plug-in.
For possible error messages (picture), "Module not found" means the targeted module of the plug-in is either deactivated or not installed. "Plug-in not compatible" indicates that the plug-in is misprogrammed and you may tell its author to fix it.
