Plug-In Store doesn't have the option for adding / trying plugins (post upgrade 2.6 -> 3.5)
If you look in your config/general.php, you will probably find that you have 'allowUpdates' =&gt; false set for the environment you are trying to trial the plugin on (stage/prod?).
Change that to 'allowUpdates' =&gt; true and you will be able to install what you want.