To make an activator primitive click-able, check the Player Activation checkbox on it's base object.
I'm thinking there's a bug in the CK, because that simply doesn't work. I copied and renamed an activator object, clicked Create Trigger, placed it in the world, selected my activator from the list, edited the activator ref in the game world, went to primitives, checked Player Activation. Trying it out in game, there is no "Activate" prompt, and the trigger does not respond.
To make it work, I copied an existing trigger ref (a mannequin trigger ref from a house), then ctrl+f replaced it with my activator. That was the only thing I've tried that has worked. I have tried making the activator object and reference from scratch, and editing each of their properties to be identical to an existing, working activator (aside from names), and it simply does not work.