Use the latest version.
Are you seeing the letterbox in preview or in the final export? What are the source and export resolutions and aspect ratio for your clips?
With the preview window set at MATCH MONITOR, test the Aspect Ratio effects choices on individual clips, to get the best presentation.

Test different export resolutions. Let others see what you see, and they will suggest specifics.
Here's how to share... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material you can link it in a personal message (PM) to me, via the envelope above. Be sure to FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to share it.