Nobody can help you without some real "hard" info like system specs. Could be a graphics error, could be something with CoDecs. Who's to say?
First thing that I would do, while gathering the info that Mylenium has asked for, is to check your video driver (Mylenium's suggestion), to see if you have the latest driver for your card. Do this from the manufacturer's Web site, and do not trust your OS to tell you, if you have the latest. Note: if on a Mac, the video driver updates will be applied, when you update your OS - update the OS.
I am having the same issue. None of the solutions I've tried have worked. Would really like to finish this lesson, any help would be appreciated
Having the same issue with my High School Graphic Design class. Has anyone found a solution? We checked the video drivers and they were current.