At what point are you importing the .xsd (XML schema definition)? Are you binding the objects located in the .xsd to existing objects in your form, or are you creating new form objects from the .xsd elements?
Once you've bound a .xsd element to a form object in LiveCycle Designer, you should then be able to import an XML data file into the form during run-time to auto-populate fields. By assigning the .xsd elements to specific form objects in LCD, you're telling the form where to populate the data when imported from the XML data file.
Shaun