The AutoCAD object model hierarchy starts at the Application object, and from there you can use the ActiveDocument property to access the Document object that represents the active drawing.
(setq acadDocument (vla-get-ActiveDocument (vlax-get-acad-object)))
The Document object has many properties that allow you to access non-graphical objects (layers, linetypes, and groups, for example) through like-named properties such as Layers, Linetypes, and Groups. Access to the graphical objects in a drawing are provided with the ModelSpace (model space) and PaperSpace (paper space) properties. The ActiveSpace property can be used to determine if model or paper space are current in the drawing.
(setq mSpace (vla-get-ModelSpace acadDocument))
(setq mycircle (vla-addCircle mSpace (vlax-3d-point '(3.0 3.0 0.0)) 2.0))