Yes, actually, Tim Brotherhood is one of our forum participants. It appears Tim make a project of providing PTC with a good basic set of material files. Furthermore, when you create new materials in Creo Parametric, you get drop-down dialogs for all that data. Therefore, the interface from inside Creo is the way to create these files but once you have a good material file for a "series" of material, it might be easier to edit the few relevant fields with an ASCII editor.
I am afraid I have little use for these capabilities and therefore have no real way to help by adding more specific information with regard to how Creo Analysis deals with the values. Check out the dialog because it really is a lot better than things I've seen in the past. For instance, it has all the allowable data ranges and units of measure already encoded into the UI. Some really powerful functions including user provided curves can be entered.
You might also contact Tim Brotherhood and see if he can shed some light on education resources rfegarding the material definition interface.