If you want to add a newsletter signup checkbox to the comment and registration form, from the Newsletter Sign-Up menu, click Checkbox Settings and check both options for Where to show the sign-up checkbox.
From the Newsletter Sign-Up menu, click Form Settings. Here you can customize the form output. It’s ok to accept the default.
To add the newsletter signup form widget, from the WordPress menu, select Appearance > Widgets.
Drag the Newsletter Sign-up Widget into a WordPress sidebar and click Save.
Reload your website page to view to newsletter widget. Create a test subscriber to make sure everything works as you expect. Remember, there are many settings for the Newsletter Sign-Up plugin and widget that allow you to customize the form and checkboxes.