The plug-in is fully integrated. Pixelan, NewBlue Titler Pro and ProDad are examples of external apps, where all of the work is done outside of PD. Most of the plug-ins don't accept any input from the timeline, however, so you can't import clips from there.
As shown in this
tutorial, you should see all of your media library content when you upen the Collage Designer. You actually don't have to have any clips in your media libray since it's possible to drag and drop clips directly onto the timeline from File Explorer, but it's easier to use this plug-in if you first import all the clips you want to use.