No one can be of much help because we don't know anything about your clip, what you did to the clip, how you set up your composition, what version of AE you are trying to use. I'm assuming you are new to AE and have never gone through the training materials and that you are using some kind of non standard video in your composition.
I'm not new to AE i had it on windows but I don't know how to use it very well on mac but thanks
It works exactly the same on Mac or Windows. What kind of footage? Where did it come from? What about your composition.