You're probably remembering an older Inkscape version. The Path Effect dialog has changed quite a lot, though I can't remember which version. Instead of the [+] [-] icons, you add effects using at the top of the dialog in a combined dropdown list/search box.
Beyond the Basics Path Effects
Inkscape.org
My path effects aren't showing up. There is no plus or minus at the bottom right.
You're probably remembering an older Inkscape version. The Path Effect dialog has changed quite a lot, though I can't remember which version. Instead of the [+] [-] icons, you add effects using at the top of the dialog in a combined dropdown list/search box.