If it's not already there, add 
using CrystalDecisions.CrystalReports.Engine
In the constructor, do the following:
ReportDocument crReport = new ReportDocument();
crReport.Load(<filename>);
ConnectionInfo connectionInfo = new ConnectionInfo();
connectionInfo.ServerName = <server>;
connectionInfo.DatabaseName = <db name>;
connectionInfo.UserID = <userID>;
connectionInfo.Password = <password>;
// set data connections for main report
SetDBLogonForReport(connectionInfo, crReport);
// set report connection for any subreports
SetDBLogonForSubreports(connectionInfo, crReport);
// view report
crystalReportViewer.ReportSource = crReport;
-------------
private void SetDBLogonForReport(
    CrystalDecisions.Shared.ConnectionInfo connectionInfo, 
    ReportDocument reportDocument)
{  
  Tables tables = reportDocument.Database.Tables;
      foreach (Table table in tables)
      {
          TableLogOnInfo tableLogonInfo = table.LogOnInfo;
          tableLogonInfo.ConnectionInfo = connectionInfo;
          table.ApplyLogOnInfo(tableLogonInfo);
      }
}
-----------
private void SetDBLogonForSubreport(
    CrystalDecisions.Shared.ConnectionInfo connectionInfo,
    ReportDocument reportDocument)
{
  Sections sections = reportDocument.ReportDefinition.Sections;
  foreach (Section section in sections)
  {
    ReportObjects reportObjects = section.ReportObjects;
    foreach (ReportObject reportObject in reportObjects)
    {
      if (reportObject.Kind == ReportObjectKind.SubreportObject)
      {
        SubreportObject subreportObject = (SubreportObject)reportObject;
        ReportDocument subReportDocument = 
           subreportObject.OpenSubreport(subreportObject.SubreportName);
        SetDBLogonForReport(connectionInfo, subReportDocument);
      }
    }
  }
}