Problem in axis selection in mechanism

Hi All

I use Creo 2

while I was trying to create a mechanism on my assembly, ( while had created the reference axis on different parts) to apply a rotating mechanism (using servo motor)

I noticed I was not able to select any axis. it seems that Creo requires me an entity and a reference. But I dont know how to select them.

What should I create on my model to create a mechanism properly ?

I have put only cocentricity constraint on my parts.