Click on the Widgets tab (1) and you should be able to see the Facebook widget (2).
Select the Facebook widget and drag it to your page (1)
Click on the widget so you can see the settings tab.
Enter the name of the page you want to set the widget, a description, the name of the site and the URL the user will like when click on this button.
Optionally, you can choose an image to be displayed in the widget. You can change also the verb you want to use and the color scheme.
Don't forget to click on Save once you finish.