Could you give this a try:
concat("<query database='clientdata' table='vwOrganizationLocationWithAddress'><columns><column name='OrgName'/><column name='Address'/></columns><filter><and><eq><column name='OrgName'/><value><![CDATA[", xdXDocument:get-DOM()/my:myFields/my:OrgNameDropDownList ,"]]></value></eq></and></filter></query>")
I put <![CDATA[ and ]]> before and after the place where the field value is added, and that should prevent ampersands from causing any issue.