No big deal. Open the file weblogin.class.php in a text editor. Do a search for "Check username for bullshit". In the very next line (line 469, if your editor shows line numbers) there is an array with a list of illegal characters.
$illegals = array('.' , ',' , '/' , '\\' , '`' , ';' , '[' , ']' , '-', "'", '*', '&', '^', '%', '$', '#', '@', '!', '~', '+', '(', ')', '|', '{', '}', '<', '>', '?', ':', '"', '=');
Just delete the ’@’, part. It should look like this:
$illegals = array('.' , ',' , '/' , '\\' , '`' , ';' , '[' , ']' , '-', "'", '*', '&', '^', '%', '$', '#', '!', '~', '+', '(', ')', '|', '{', '}', '<', '>', '?', ':', '"', '=');
And now you’re a programmer