Left-click points around the model (on gingiva and/or teeth) to define the margin line.
In the surface properties section there are 3 sliders which allow you to define the thickness of the different areas and the outer smoothing of the splint.
Occlusal- The thickness of the occlusal surface of the model
Peripheral- The thickness on the axial walls of the model
Smoothing- Controls the smoothness of the top surfaces of the splint. The larger the number, the smoother it will be.
After the parameters are set click Apply. The next set will take you to "freeform the bite splint top" which is similar to free-forming, see Freeforming "Free" tab.