Tap on the video clip and then add the theme by tapping the little sprocket before your video clip. Then, when you have your theme up, double tap on the clip and then it will bring up your editing options. And when your theme is applied, you can tap inside the "text boxes" in your video and it will bring up the text editor. It takes a little playing around with to get used to. Personally, i dont think it was worth 4.99 though...
🙄
Here is a sample video of what i did on youtube...
Video Clip Using iMovie (720p)
Hope that helps!
-Jason