Select the objects that you want to include in the model. The objects must be siblings; that is, they must be at the same level and share a parent node in the explorer hierarchy. Any children of the objects you select will automatically become part of the new model.
If desired, you can also select any groups that you want to include in the model. However, any objects that are members of these groups will not be automatically added to the new model unless they are also added to the selection explicitly.
Click Create 
Model on the Model toolbar. The Model property editor opens.
A null is added to the scene as the parent of the selected objects. You can add other objects to the model at a later time by using the Parent button on the Constrain panel or by dragging and dropping them onto the model's node in the explorer.
When you merge a scene into the current scene, it is automatically made into a model. This ensures that the names of elements are preserved: because each model maintains its own namespace, there is no need to append element names with numbers to make unique names.
In the browser that opens, locate and select a scene, then click OK. The Model property editor opens.
If you don't want the elements of the merged scene to be a separate model, you can "unparent" the model's children using either the Cut button on the Constrain panel or by dragging and dropping their nodes in the explorer, then deleting the model node.
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in an explorer.