How do you make a sphere? I tried making a prop_dynamic with the world model mp_ball.mdl, but it doesn't show up in the map.

The entity npc_personality_core.

I mean the sphere that you can put into a sphere button. That's just a personality core that does not work on that.

Use a prop_weighted_cube and set it's object properties with Cube Type = Sphere and Use New Skins = Yes.

oh, thanks. I didn't notice the Cube Type line.

If you really want to use a prop, it needs to be the right type. Since the player can pick up and play with the sphere, it would have to be a prop_physics. P2 knows that, so it kills it if you give it the wrong type. Sort of a way of making sure one mapper doesn't confuse everyone by making a physics chair, when every other chair in the game is dynamic.