Yeah, so apparently it was just an installation problem. I actually had this app on the IPB 3.4.8 version, so I just upgraded it. My broser crashed during the upgrade and it showed some sort of error, so that must have affected the installation. I uninstalled it and re-installed it again, and now it works.
However, I can't figure out how to set the restrictions for which groups to use the app. Also, is it possible to poke the same member several times in a row? Because once I poke a member once and refresh the page, the poke button disappears from their profile.