"Does a mediator more or less referee the couple's disagreements to come to a compromise ? Or can they also 'add' more to the lists of discussion points ?"
Yes. The mediator is a qualified neutral who tries to broker agreements. That may mean plying Devil's advocate, indicating his/her idea of what a Judge might do, pointing out the cost of litigation and the risk of court and more. For child suport, the calculator is what a COurt will use - so if the parties disagree, it is really a non-issues since the calculator will be what happens. In other words, there is very little leverage with regard to child support. A good mediator may suggest things the parties have not considered, but it is certainly ot their primary role or responsibility.