Séverin is right. On the plugin front page, you are presented with a Download / Install button that matches your Moodle version. The Plugins directory picks the latest plugin version that declares support for your Moodle version. If there is no matching version found, no button is displayed and you have to go to the Versions tab and pick one youself.
To change your Moodle version, use the selector at the top right block area (see screenshot attached).
