Following this list (which is initially empty) is a list of applications which support includelets and are available to be mounted. Note that you can mount as many instances of each application as you want. This makes sense for some packages (content-includelet, for instance), but not for others (file-storage). You'll need to depend on your knowledge of each application to decide when it makes sense for you to mount a package more than once. Note that this is no different than when deciding how many instances of a package to mount when configuring a "normal" subsite outside the context of the Layout Manager Integration package.
When you mount an application, it will be listed in the top half of the page, with no layout elements bound to it. Click on the Manage Includelets action link. This is described on a separate documentation page.