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 2.3.6 (8 October 2009) has been released. Amongst other things, the bug where articles with no menu links were sometimes given the wrong Itemid has been fixed.