Log into the site editor and click the 'Insert' tab.
Under the 'Apps' heading in the left sidebar, choose 'Pinterest Pin It'.
The widget box will appear on the page. Nothing else needs to be added to the widget - just click and drag to place the button where you'd like it to appear.
Click 'View my site' in the top right corner to see how the button displays on the page.
Clicking the button will pull all images from the page and from there visitors can choose an image to pin.