In FCP your timeline and your footage need to be anamorphic, if you capture the footage without this setting you can change it in the bin just be clicking the anamorphic field it the columns. This is one of the only things you can change after capture but you need to do it before you put it in your timeline.
When you get to compressor you need to make sure the aspect ratio is set to 16:9. Then when you put it in a track it should be set up as 16:9 Letterbox. Not 16:9 Pan and Scan or both, unless you have edited in a way that if the left and right are cropped off the full screen 4:3 image is still okay. The Letterbox or Pan and Scan after the 16:9 tells the DVD player how to handle a 16:9 track if it is hooked up to a 4:3 TV.
[Joanna8555] “absolutely no letterboxing”
The only way to guarantee this is for you to get a guarantee from every viewer that they will only view it on a 16:9 TV. DVD players will automatically letterbox an anamorphic movie when hooked up to a 4:3 TV.