You must enter the all the 4 - DB_NAME, DB_USER, DB_PASSWORD and DB_HOST.
enter your database name for Pligg, enter your username for the database, enter your password for the database and last where the database is located and also pls do change the following line too
Code:
$message ="http://localhost/new/pligg19/validation.php/?code=$encode&uid=$username";
to point to the location of you validation.php file on server.
Sorry we had it hard coded, will fix such that it picks dynamically; on Monday - currently out of office and no access to server.