How to end a workflow or go to current activity via existing smart objects?

public static string GoToActivity(int processInstanceId, string activity)
        {
            string result = String.Empty;

            try
            {
                wms = OpenWmsConnection();
                wms.GotoActivity(processInstanceId, activity);

                result = "SUCCESS";
            }
            catch (Exception ex)
            {
                result = "FAILURE: " + ex.Message;
            }
            finally
            {
                wms.Connection.Close();
                wms.Connection.Dispose();
            }

            return result;
        }

 private static WMS.WorkflowManagementServer OpenWmsConnection()
 {
 var wms = new WMS.WorkflowManagementServer(//pass your details here);
 try
 {
 wms.Open();
 }
 catch (Exception ex)
 {
 }
 return wms;
 }

Above is a custom method I wrote and exposed as a smartobject method. You pass in the activity where you want to go and the processinstanceid and it does the rest. I have removed some stuff thats not related but the above should work for you. Hope that helps