Open the file (2D)
As soon as 3D is introduced into the file a managed 3D model is / will be created.
3D can be introduced into the file by attaching a Terrain as a reference file and make it active. It could also be simply profiling a 2d element.
Once the 2file has a managed 3D model, setting up the view is easy
1. Open 2 windows in MicroStation IE window 1 will leave as 2d window 2 will set to 3D)
2. open the view attributes on window2 (top left corner of the window)
3. Expand the view set up and set the model and change the model to the managed 3D view (IE Default-3D)
HTH