How to add new objects
The Create Object mode allows to add new level entities to the scene (Sky, Clouds, Lights, Sound Emitters, LUA Triggers, etc)
To add an object, first enter the Create Object mode by clicking its button. An additional toolbar will appear below the main one: from there you can select the type of object, finally click on the 3D Viewport to add the select object to the scene.
New objects are added on the same level of your current selection on the Scene Tree. If nothing is selected, the object will be added to the root of the tree (bottom)
Other SimObject classes
You can choose to create other entities that are not visible in the main selection using the Other Classes button.
From there you can choose a class, search for it, and press the Create
button to create an instance of it, make sure you select a folder or a
sibling in the scene tree window, otherwise it will be added to the root
of the tree (bottom).