FilterService Find method

I'm trying to filter on a Process Data field or Folio. I want the user to provide me a value (such as the document name) or Folio value and be able to get the Process Instance ID from that search in order to display the workflow status.

Below is my test code. Nothing is being returned in the dataset. What am I doing wrong?

string strXML = "<?xml version="1.0" encoding="utf-8" ?><Search><Criteria>CR-155</Criteria><Scope><Folio Search="True" /><Folder Search="False" /><ProcessName Search="False" /><ActivityName Search="False" /><EventName Search="False" /><ProcessData Search="True" /><ActivityData Search="True" /><Originator Search="False" /><Destination Search="False" /></Scope><Filter><DateFilter Filter="False"><FromDate /><ToDate /></DateFilter><ProcessFilter Filter="True"><Folder>ECM</Folder><Process>ECM</Process></ProcessFilter></Filter><Sort>Folio Asc</Sort></Search>";
System.Xml.XmlDocument findDoc = new System.Xml.XmlDocument();
findDoc.XmlResolver = null;
findDoc.LoadXml(strXML);

K2FilterService.FilterService wsK2 = new K2FilterService.FilterService();
wsK2.Credentials = new System.Net.NetworkCredential("username", "password", "domain");
System.Data.DataSet ds = wsK2.Find(findDoc, @"domainusername", 5);