@jamesh. I really do not understand what you have stated in your last post. The only version you should install is the latest version. You did not need to uninstall the previous one.
What version of PHP are you using? You need PHP 5.3 in order to use the component.
You should never use older versions of any software. If you were using version 1.5.5 previously, upgrading to 1.5.6 won’t provide any problems for you.
In addition, there is documentation on how to use the Companion plugin. The companion plugin is an optional feature that allows you to add a listing to an existing article.