When you say "buttons" you mean buttons on your actual computer keyboard, or keys on a midi keyboard?
Sounds like the plugin is stealing input focus from your DAW, which makes it so you can't use the buttons. FL Studio has a button for this on the plugin's toolbar. However, this is definitely not a problem exclusive to Nimble Kick, I've had this happen with many of the plugins I use. I'd recommend you get a midi keyboard to work around this problem.