go to the bindings panel and click the plus button, select URL variable and set the name to "file"
create the link as:
<a hef="downloadpage.php?file=<?php echo $row_recordsetname['filenameColumn']; ?>">Download</a>
where downloadpage.php is the name of the page with the download behavior and behavior on it, recordsetname is the name of the recordset and filenameColumn is the name of the file name column in the database.
in the download behavior, clickthe lightning bolt icon next to trigger and filename and select the file querystring binding.