. . . . How do I apply the widescreen black bars to my videos . . . .
Movie Edit Pro (MEP) will automatically apply the black bars top and bottom if you add 16:9 aspect ratio (AR) video or images to a 4:3 AR project - ie Letterbox the video/image.
Similarly MEP will automatically apply the black bars left and right if you add 4:3 AR video or images to a 16:9 AR project - ie Pillarbox the video/image.
If the video or image aspect ratio is the same as the project then black bars will not appear - they are not necessary.