Click an image on the layout and the Aperture panel will open on the left side of the screen. You will zoom and crop in this side panel, not on the layout itself.
Use the scroll bar under the featured image to zoom (crop) into the image. Click and drag on the image in the side panel to reframe it.
Tip 1: It's good practice not to crop in too tightly where images bleed to the edge of the page (albums are guillotined in construction and you will lose a couple of millimetres from the edge of the print).
Tip 2: For similar reasons you should keep important image data away from the edge of apertures in matted albums.
Tip 3: It's always good practice to keep important subject matter (e.g. faces) away from the centre (spine) of layouts, even in panorama Flushmount albums, where the prints are creased, not cut at the spine. Classic Flushmounts have a 2mm gap at the spine (gutter), as do all matted albums (larger offset-printed Flushmounts have a smaller gap). Finally, some image may be lost in the spine of perfect bound Photo Books.