Originally posted by wolfiegirl I think you will find that is the opposite of what you need - it adapts a 1/4" screw to a 3/8" hole, and you need to adapt a 3/8" screw to a 1/4" hole.
The three options as I see it:
1. An adaptor like the Desmond product UncleVanya linked to.
2. A dedicated head like
this which is the one I use with my SIRUI monopod. It lets me tilt the camera relative to the angle of the monopod, which is sometimes useful.
3. Learning how to use the monopod directly with a camera. If it's a Manfrotto, there is a good chance it natively supports both 3/8 and 1/4 thread somehow. Either part of the screw pushes down on a spring to reveal a narrow screw within, or the top of the monopod detaches and can be flipped over, or something like that. If you post some close up pictures of the monopod we might be able to figure it out with you.