Question about Simple Post Thumbnails made by Press75.com

I have been search online all day and have decided to post a question here. I am new to WP (just started today actually). I am using the theme “Video Elements” v2.0 from press75.com. I install all the plugins that it asked me to (which included Simple Post Thumbnails v1.5). My problem is that when go to make a video post and try to add a thumbnail picture, it won’t show up. I have tried everything I can think of. If you know how to solve this problem, PLEASE HELP. Thanks.