I recently purchased this item, and attempted to attach it to my 70-300 Canon lens. It does not fit. Is there another item required to mate these two items with the usual Canon elegance?
Is your problem that you can not attach the extender to your Canon 70-300 lens? Does that mean you can attach the extender to your camera? And you can attach a different Canon lens to the extender? I purchase this item in 2009, and was able to use it right out of the box. No other item was required to mate this to my camera and lens, including my 70-300. Simply lined up the red dot to the camera body and turned counter clockwise as if the extender was a normal lens. Then line up the red dot on the lens to the extender and turned counter clockwise as if the extender was the camera body. Sorry that I could not be more help.