MotionBuilder provides the following methods of adding and loading scenes and assets:

• Select File > Open or File > Motion File Import from the MotionBuilder menu bar to select a file using the file browser.
• Select a previously saved file from the list of recently opened files in the File menu.
• Drag an asset or .fbx file from the Asset browser into the Viewer window or Scene browser.
• Select Merge to merge a file with the current scene.

  See Merging.