All objects forming 3D scene are displayed in Tree of objects. New object, which you want to add, can become a parent (container for other objects) or child of any object. Therefore you must choose necessary place in object tree by the left mouse button. For example, if you have chosen Easy 3D Objects and then have chosen the necessary object from menu item Object / Add, that added object will be child of Easy 3D Objects. If you have chosen other object in Tree of objects then added object will be its child. Your added object can become parent for objects which will be fasten to it later.

Also it is possible to press on the chosen element of Tree of objects by the right mouse button and to use popup menu item Add.