You can access this tool from the following:
This tool sets the active terrain model file. Default terrain model is the model which is displayed by default in profile models and is the default target for corridor modeler.
If the end conditions of a template within a corridor interact with existing ground, then an active terrain model is required. Only one terrain model can be active at any given time, however, multiple terrain models can be combined into a complex terrain model. The terrain model can be in the same design file as the corridor model, but more likely is in its own file and referenced. Locate Set Active Terrain Model tool in the Terrain ribbon.