It sounds like you are on the right track.
If you're not getting any PHP errors, there are a couple of things you can check.
First, try echoing your parsed parameters to make sure they are getting through to your script and are thus included in the query.
Check the way you access your database. If you're using a different database than Wordpress, you'll need to make sure your connection is set up properly. From the Codex:
The $wpdb object can talk to any number of tables, but only to one database at a time; by default the WordPress database. In the rare case you need to connect to another database, you will need to instantiate your own object from the wpdb class with your own database connection information.