Putting ringtones on the iphone 4s via itunes

I've several ringtone files (m4a) and I've plugged the iphone 4s hooked up via itunes. But I can't load the m4a files into itunes.

I've tried file > adding to library and nothing happens. And simply dragging the file onto itunes in various folders on the left-hand side.

But I get the error message: "ringtone.m4a" was not copied to the iPhone because it cannot be played on this iPhone. This seems odd because I know the files are good.

Any suggestions or tips or links? Thank you!

PS. I don't even have a ringtone folder, something others have reported having on their itunes:

On left hand side menu of itunes I have:

Devices

>iphone

Music

Movies

TV shows

Books

Purchases

Voice memos

But shouldn't there be a folder available to drop ring tones in?