Which object model are you using?  CrystalViewer or ReportDocument?  In either case, you have to access the Parameters property of the report.  It looks something like this:

ParameterDiscreteValue discreteVal = new ParameterDiscreteValue();
ParameterValues curvalues = new ParameterValues();
foreach (ParameterFieldDefinition parafld in crReport.DataDefinition.ParameterFields)
{
  if (parafld.DiscreteOrRangeKind.ToString() == "DiscreteValue")
  {
    discreteVal.Value = Request.QueryString[parafld.ParameterFieldName];
    if (discreteVal.Value != null)
    {
      curvalues.Add(discreteVal);
      parafld.ApplyCurrentValues(curvalues);
    }
  }
}

You can't do it inside the report.

-Dell