I found this post:
I created the randIndexPP attr, (data type Vector and Attribute Type Per Particle).
I can see that in the attribute editor, under Per Particle Array Attributes the Rand Index PP is there. I right click it and write a creation expression for it (particleShape1.randIndexPP = rand(0,2.9);) I understand that under ParticleShape1 instancer i have to change the Object Index from "None" to "randIndexPP", the problem is that it doesnt appear there... What am i doing wrong? I tried starting the scene from scratch and have the same problem.