in order for you to be able to add an .htaccess file to a domain at dreamhost that domain must be “fully hosted”. There is no place to put an .htaccess file for a domain that is not fully hosted.
“fully hosted” means the space on the server is created, apache is pointed to read that space and serve it on your named domain.
things you can do other than “fully hosting” include redirecting, parking etc. and since there is no direct server space–there is no place to put an .htaccess file for those things.