To get a model into your scene, open the main scene, then go to File - Merge, and select the object (chair for example) you want to merge in.
If the chair in this example isn't setup for Corona, you need to run the Corona Converter Script and with that object selected you need to convert materials for that object.
If you want to save an object from an existing scene, select the object, then go to File - Save Selected.
That's should be a good start for you.
To find the Corona Converter Script, with you scene open and model merged in, go to Scripting - Run Script and assuming you haven't changed any installation paths, it will be found under the Corona Scripts Folder.