Hi,
Have you studied the samples provided with this playmaker bridge pack from the wiki? in there you see how the "speed" property of the anim graph is controlled.
The "problem" with this technic is that it's one of many ways to get it done and very much something to deal with on a per character, per mecanim, per project need... so not real hardwritten rules as to how to control this "speed" property unfortunatly. So indeed, like you did, it's better to understand what exactly the mecanim graph is expecting in the first place ( yes, "speed" is misleading in this case, but I tried to por the Unity samples as they are truly made, so I did not change anything on naming, protocoles and features, so that you can really study a true Unity example made with PlayMaker).
bye,
Jean