1. Create a duplicate track of the image to be pixelated
2. Drop a cookie cutter on the top event. Size it.
3. Drop a pixelation, blur, or other obscuring filter on the top track.
4. Done, but you'll still need to keyframe the cookie cutter's motion over whatever you want to hide/blur.
You don't need to use track motion for this, keyframe the cookie cutter target.
if you really want the pixelation/blur to be obvious, add a brightness filter to the same track and brighten it up a little. Makes the pixel shifts more sexy.
There are also a couple other methods of doing this, this is quick to describe.
You'll find a couple veg's on the Sundance site teaching you how to do this.
Copy the video on to 2 tracks, on the top track apply a cookie cutter for the size of your logo or plate make sure in the settings to change the method to Cut Away Section. On the second video track apply some kind of Blur filter. You will now have a little box that is blurred on the screen. IF you go back to the first track you can then ketframe the box to move around and follow the image you are trying to cover up.