How to create Force output with expressions

Hi, 

I have a simple spring - mass system where a Force = 1 N is applied on the CM of the body. 

I'd like to plot the stiffness, but I don't know how to do it. 
I can't plot the force either. I create output, select expressions, in F2 I type the expression `FX({pmb_0.cm.idstring})`.

I select the CM marker of the body but the output is zero. 

If I use the predefined Force output I can see that the force is 1 N, as it has to be.