You need to download the plugin from JoomlaCode and install it. Once you've downloaded it, go to Administrator » Extensions »Install/Uninstall (in your Joomla! web site) and install the plugin. Make sure to activate it in Administrator » Extensions » Plugin Manager » Editor Button - Linkr.
Linkr 2.3.8 (13 June 2011) is ready for download, and is compatible with MooTools 1.2. It has also been tested with the Firefox 4, Internet Explorer 9, and Google Chrome browsers.