1.To enable search engine friendly URL, you need to Install the SEF

2.YOu can download the SEF from here

3.http://www.oscommerce.com/community/contributions,2823

4.Download the software and extract it in your local computer

5.Then you should upload the files, to the OS commerce directory.

6.Modify the .htaccess file and add the below line into it

RewriteBase /catalog/

7.In admin /includes/languages/english/categories.php file, before the final ?> add the following lines

define (‘TEXT_PRODUCTS_SEO_URL’, ‘Products SEO URL:’);
define (‘TEXT_EDIT_CATEGORIES_SEO_URL’, ‘Category SEO URL:’);
define (‘TEXT_CATEGORIES_SEO_URL’, ‘Category SEO URL:’);

8.The final step is to update the database and execute the sql queries.

9.Now log into the admin area under configuration, select SEF URL’s and click on the configure button, to configure it.