Thank you for your answer. I numbered the equations in the question at :

http://stats.stackexchange.com/questions/78167/confusion-in-expectation-propagation-energy-function

Here are my problems now:

(1) I didn't mean the connection between primal and dual energy function. What I want is the way PRIMAL is derived. (Equation b, in the link) is the primal, and we know evidence is (Equation a, in the link), which is
exactly the same as (Equation c, in the link). Now the question is how (Equation b, in the link) is related to (Equation c, in the link)?

(2) Oh, I misunderstood. Then what equation do you solve to find $s_a$
? In other words, what do you exactly mean by "the scale factor which minimizes the KL-divergence"?