Add simulable objects to Photo Studio

Hello

I'd like to create an animation with a studio photo. To do this, I created an experience.

On the other hand, I can't add simulable objects.

Do you know how to do it? 

For the moment, I am doing the following process:
1) Double click on the experience
2) Edit simulable objects
3) Add
4) A selection window opens. Now I try to click on my assembly or parts but nothing is selectable...

Thank you in advance for your ideas.