Open your /serendipity_genpage.inc.php file and add the following line inside the <head> section:
This will allow you to make a file called keywords.txt and place it inside your Serendipity root directory. This plain-text file can contain your comma-separated meta keywords for particular search engines such as Overture and others.<meta name="keywords" content="<?php include('keywords.txt'); ?>">
Just thought you'd like the suggestion