Create a multi-bosy part
 

Hello,
I am currently trying to make a 2D model of a Carbon storage saline aquifer. The aim is to monitor CO2 migration to the ocean surface. To begin I am only doing a few layers. The first layer will be the ocean, the second layer will be a porous medium representing the earth's crust, and the third layer will be CO2.
So far I have sketched the geometry and created 3 parts, I have then mapped face meshed, and named each layer/part as well as the ocean surface that I have called OUTLET.
I hope that when I reach setup I can assign separate materials to the separate bodies, would I be right in thinking this?
On loading setup I get an error "Model Information is incompatible with incoming mesh". Do I have to create a multi-body part and if so how do I go about that? I have seen on other threads that you just right cvlick the tree, but I only get the options:
Analysis Tools
Suppress Surface Bodies
Generate
Any advice would me more than appreciated!