After much frustration, reading, and googling, I finally got the magic incantation to add a Swift class to my ObjC code. You have to create the Swift class and eliminate all errors, then delete it from your project (not to trash) and drag it back in. Then, finally, I got it to add the magic interface file so it could be accessed.
Another point -- The docs lie. The Swift class absoultely has to inherit from NSObject, or it will not allow you to add @objc to the file.