dynorod
If you already have your parameter created:
1) Open the parameter by double clicking on it
2) right click in the value area box
3) On the contextual menu, select Edit formula. This will bring up the formula box.
4) When the formula box appears, you can insert differnt equations, or you can go to the catpart, and select the parameter from there. Since you want material properites. Select the material in the catpart. From there, you can choose what you want from the part definition.
Remember, units matter in formula creation.