Please use the VST3 of Natural Drums in FL Studio as the AU version has an issue with looping.
Using Natural Drums as a Virtual Instrument
Add N-Drums as a new instrument track by either dragging it from the plugin browser onto your channel rack, or via the Add > Other plugins… menu at the top of the window.
Using Natural Drums as a MIDI FX
FL Studio does not host MIDI FX plugins in a conventional way, but a workaround can be applied.
1. Once you have rescanned your plugin list, find N-Drums MIDIFX and change its type to Effect
2. Create a new track using your 3rd party plugin of choice, and insert N-Drums MIDIFX as an effect insert on that channel by dragging and dropping
3. Go to the settings (the cog icon on the top left) of this instance of N-Drums
4. Change the MIDI Output port to a value of your choice
5. In the same settings page of your 3rd party instrument plugin, change the Input port to the same value as the N-Drums Output port
6. Natural Drums will now stream midi into this plugin when FL Studio is playing