Starting with version 2025 Particle Illusion shows an "Add" button when the mouse cursor is over any emitter in the Library View -- clicking it is the easiest way to add an emitter to the Stage. You can still use double-click, but on some Windows systems a triple-click might be needed -- in that case the "Add" button is the best approach.
You can also Alt-Click on a library emitter to add it to the Stage, or if the “Click in Stage to Add Emitter” Preference is checked (in the Preferences Dialog) each click in the Stage will add the currently selected emitter.
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