Setting up a WordPress MultiSite Install to Use Multiple Domains

Hello, I have just purchased your multidomain plugin and before I install that, I am trying to install multi-site on my wordpress. It seems I have done something to my wp-config.php file to make it not work, and I am really a beginner. My wordpress is currently installed under my blog, My goal is to be able to manage a separate site for my graphic design business,, and to do some testing on sites with different domains for new web design customers. I really need someone to hold my hand through this and would appreciate help!