The first problem is that your first component in the assembly has no constraints. This is bad for any assembly and even worse for a kinematic assembly. The first model added to the assembly must be grounded and fully constrained. To do that, just edit the placement and add the Default constraint. Your second model is assembled properly.
Looking at the servo motor assembly, it appears you are using the generic assembly which includes both the metal and plastic horn. You should change your assembly to only use one or the other, but not the generic since it contains both designs. This can be done quickly by using the right mouse button option for Replace and choose one of the instances (metal or plastic).
To define the motor, you need to enter Mechanism mode and define the type of motor you'd like to create. I was assuming your question was about defining and saving the motor definition in Mechanism mode, but maybe it was due to the sporadic behavior of your assembly with the missing default constraint for the first component and the competing pin joint connections in the generic servo motor.
I'll upload a new copy of the updated CAD model to your document.
