This was difficult for me at first too. After I place an object in a terrain scene, I scale them larger if needed, so I can see them in the Camera view relative to other objects or terrain(especialy in Large scales, like terrain presets). I use the commands; "Zoom to selection", "Reset 2d pan", and "View selection", plus "Send to Origin" alot in a 4 viewport setup to help with placement and finding locations/objects.

 When I get the object I want placed, the "Scene's Magnitude" settings can then be used for better/finer control(like Camera speeds etc). I worked slightly different 3dcg workflows and the scaling in Carrara threw me off for a bit, but now its almost second nature to set things up. Feel free to ask questions, cause there are many more workflows to manage the scenes and presets in Carrara. Hope this helps.