Hi Jack,
A few more points on adding a surface region or volume region to an assembly.
1) These features are added to a component not the assembly. Once you select the component, you can sketch your form and then choose the surface to apply the the surface region too or define the depth for the volume region.
2) In the event you have an assembly cut created which intersects the component in which you are trying to create a surface or volume region on, you will be presented with the message you attached in the initial thread. Creo Simulate does not allow creating simulation feature such as surface regions or volume regions on a component that intersects with an assembly cut.
Hope this helps.
Thanks,
Mark