You first look for the plugin you wanted (in this case TMXImporter), using the forum search
In this case, TMXImporter is here
construct.net/en
After you download the zip/7z file, you extract it and put it in the plugin folder which is
C:\Program Files\Construct2\exporter\html5\plugin
After that you restart Construct 2, and you can put TMX importer in the canvas...
Edit: Oh, and the manual entry for installing addons is here
construct.net/en