To import a .docx file, choose File > Import > Files, then navigate to the file you want to add to your project. Scrivener imports the file and adds it to the Binder. If you have selected the top-level folder in the Binder - such as Novel, Manuscript, etc. - Scrivener adds the imported file just below that folder; if you have selected another folder in the Binder, Scrivener adds the imported file to that folder. You can, of course, move the file to a different location in the Binder.