Photo Mechanic applies a non-destructive crop to photos, meaning that the original version of the photo is still available when you export cropped versions.
You can find the crop tools in the side panel of the preview window or simply press the C key to toggle the crop cursor on and off.
When cropping, you can resize crops by dragging on a corner, move the cropped area by dragging inside the selected area, and rotate by dragging outside the selected area. When rotating, you will also see a grid to help you align your crop with the image.
Preview your crop by either using the "eyeball crop" button in the crop tools or by using the P key to toggle preview on and off.