Go to the site editor and click 'Insert' in the main toolbar.
In the left sidebar, from the 'Apps' category choose 'Facebook Like'. The app will be added to the page.
Double click on the app to open the app window, and paste the URL of the page to 'Like'. You'll see a preview of the app on the left of the window. Click 'Apply' and save your changes. You can preview the app on your site by clicking 'View my site'.