Get rid of the text that's here and create a new text box.
Let's shrink down this so we can see what's going on here. So let's go, in Keynote, and create a widescreen presentation because the video is probably going to be widescreen. However you could do it pretty easily in Keynote. But if you want something a little more complex you probably need a piece of animation or graphic software. You can use that in end credits for a project.
Video Transcript: You have the ability in iMovie to create simple scrolling text. Check out Creating Video Scrolling Credits With Keynote at YouTube for closed captioning and more options.
