Your footage is currently "anamorphic," but you need it to be in "letterbox," meaning that it is a 4x3 frame with black bars above and below the 16x9 image.
You'll be able to do this using the Video Filters. Search the help file/manual for instructions. It is your best friend, and the FCP manual is very thorough.
You could create a sequence that is 4x3 and put the footage into that timeline. Once you render it on the timeline, you can either work on it directly within the sequence OR export a new 4x3 quicktime and re-import it, depending on what you need to do with it exactly.
It would help us to know more about what you want to do with the footage (crop, letterbox, export, etc) so that we can give you the right answer.