On Mac, you might need to add Nimble Kick to your system plugins path, as FL Studio doesn't automatically add the user's local plugins folder to its plugin manager.
To get to this location in Finder, open a Finder window, then in the top bar click on "Go" followed by "Go to folder" (or press Shift+Cmd+G), then type "/Library" (note the slash at the start). From here, navigate to "Audio" -> "Plug-Ins". In here, you might need to create a "VST3" folder if it doesn't exist yet. Inside of this folder is where you place Nimble Kick.vst3.
Then, open FL's plugin manager and re-scan for plugins. Nimble Kick should now show up.