Adding Reactions is done through the settings page of the Blog Post widget. From the Layout | Page Elements tab, simply click the 'Edit' link under the Blog Post widget and a settings window will pop up.
The Configure Blog Post window will display a handful of options, and the Reactions feature is near the bottom of the list:
To turn on Reactions just select the checkbox, and then you will be able to customize the feature by adding words of your choosing. Finally, you can choose the exact location of Reactions by dragging the widget at the bottom of the Configure Blog Post page: