I assume you're working with an image and not a video. The first time you use the Pan & Zoom tooland Motion Designer, you should have seen a prompt to watch a
tutorial. The one it's linked to is actually from PD7, so the interface is older but the basic fuinctionaility is mostly unchanged.
There are several ways to zoom in on a clip and stay there, and if you want to use the Pan & Zoom tool, chose the
Zoom In preset. It will start partially zoomed in and will then zoom in near the top left corner, and if you want to change the location and/or amount of zoom, click on the
Motion Designer button at the lower right of the tool window and make the desired changes.
If you want to start with the full screen view, change the size and location of the viewable screen at the starting keyframe. To change what the zoomed in clip shows, click on the ending keyframe and drag the window and adjust the size to be centered on your desired object. PD will automatically calculate all the intervening changes.