Help With Atomic Particles & Audio Interaction?

Hi, 

I'm new to HitFilm, but I'm trying to set up some Atomic Particles and get them to animate in response to audio. 

No matter what I do, when I put the Atomic Particles in my layers, I never get an option to add an audio layer under 'Audio Interaction' - just the mapping settings, but no way to choose an audio layer. 

Is something wrong with my installation or am I missing something dead obvious? Any time I've seen a tutorial video, the option is clearly there. Help!? 

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