You need to be signed in with the same Google account that you use for Paperpile. Otherwise the add-on does not know who you are and can’t show results from your library.
You should see the same results as you see when you use the “P” icon in Google Docs that brings up the citation dialog in the middle of the screen.
Please test with a search term which you are sure that it exists. Also if it’s already cited it’s now shown again under “My library” but only in “From this documents”