Hi,

The function you're looking for should be under the "Movement effects" section ("Effects" tab in the media pool). Make sure you've selected the object you want to manipulate in the timeline and choose the "Section/Zoom" facility under the "Movement effects" heading. In the preview monitor, the rectangle will now show up that encloses the part of the picture that will eventually be visible. At first, this will obviously be the entire picture. You can use the orange handles to resize the rectangle and crop down to the section you want to see.

I hope this helps.