As pointed out by @SamanthaNguyen:

You'll need to set the following in your LocalSettings.php (see https://www.mediawiki.org/wiki/Extension:SocialProfile#Board ):

$wgUserBoard = true;
$wgUserProfileDisplay['board'] = true;

This will add the UserBoard link to the User page like this:

Edit profile | Upload avatar | My watchlist | Contributions | User page | UserBoard

The UserBoard show sent private messages and this is an important feature of this extension. I suggest that you show this link by default so people can navigate to their inbox. When its not visible people tend to forget how to access the UserBoard at all.