Hi there,

Applying an actual video effect (e.g. mosaic/pixelation) to only a selected area of the video footage is not possible. What you should probably go for is basically putting an overlay image/object over the part of the video you want to hide. You can basically just take any image and use the program's facilities to position and resize the image to get it over the right spot. Chances are, you will also have to add a movement to reposition the image if there's movement in the original video. If it's a somewhat static scene and the part you want to cover stays in the same position, that would obviously make it much easier. I'd suggest just using one of the decoration elements provided with the program as the overlay image. You will find something in the mediapool under the "Effects" tab => "Design elements" => "Image objects" => "Deco". There's a simple black bar, among other things, which you can pick and then simply position and resize on the image.

I hope this helps!