You install and manage plugins in the Plugins Tab. This tab shows the installed plugins on the left, and detail for each plugin on the right. At the top right, you'll find the 'Add Plugins' button that lets you search for plugins and install new ones. You can sort plugins by ratings, usage or submission date. As the Ecosystem grows ratings are important to maintain the health of the plugin ecosystem, see below regarding reviewing plugins.
Once you've installed a plugin, click DONE and you'll see the newly installed plugin in the Plugins Tab.
Many plugins, based on external services, require you to enter some keys to identify your app with them. For instance, for analytics plugins, the key will be used to identify your app. You can enter these keys in the Plugins Tab.
Keys are usually shared among the Development and Live versions of your app, but if you need to use different keys (for instance so that development sessions don't interfere with production analytics), you can enter a development version of your key. If a development key is provided this one will be used in development version of your app, otherwise the live key will be used.
This view also shows the different items that the plugin is adding to your app. Elements, API calls and actions will be listed. In general, you'll find API calls in the API provider dropdown for the 'Get data from API' palette, you'll find elements in the New Element palette, and actions will be in the Plugins section of the New Action element.
Lastly, you can uninstall the plugin of your app. Note that adding plugins adds some code to your app and can make pages heavier and slow page loads down. Try to keep the list to the necessary plugins you need when running your app.
As a way to moderate the marketplace, ratings and users' feedback are very important. You can rate plugins in the App Editor (and please do so!). Ratings are communicated to the Plugin Builder via the Statistics Tab in the Plugin Editor, and will be useful for them to improve the plugin.
As each plugin is different, we recommend using the support of the Community through the forum for help to use a plugin.