In your current model, create an empty submodel. This should be fairly large, so that the diagram of the model you will be inserting is not all squashed up.
Select
pointer mode, and double-click on the boundary of the submodel. (If you miss, you will see the submodel properties dialogue window: cancel this and try again). You will see a new model diagram window, just like the main desktop window except smaller. It will have the same menu as the main desktop window.
Select the "Open" command from the "File" menu, and locate and load the Simile model (.sml) file.
You will see the loaded model in both the new submodel window, and in the round-cornered submodel box in the main Desktop window. You can close the submodel window, or keep it there if you want to work on the submodel and it is too small in the main Desktop window. Note that, if you enlarge the submodel window, you will probably want to do Zoom to fit in order to get the model submodel diagram to occupy the whole of the available space.
In: Contents >> Working with submodels