Go to the backend and choose components > add user frontend
Click on the Parameters button
Search for the parameter: "Email notification" and configure it.
If the parameter "Email notification" is missing there, you probably have an old version of the component. Due to an error in the translation of the website the English download page of the component was still serving the previous version of the component.
Go to:
http://www.ghanja.be/en/downloads/components/add-user-frontend/details.html
And install the latest version. Which is: add user frontend 1.2.2