Hi Arjuna,

 Have a look at the help files, or the developer reference/SDK it is full of sample code and examples of this. The following is a slightly modified version that should get you going...

public void ActionWorklistItem()

        {

            // TODO: Replace these placeholder values with values for your environment

            string _serverName = "blackpearl";

            string _user = "K2Student";

            string _domain = "DENALLIX";

            string _password = "K2pass!";

            string _wfSerialNumber = "4_12";

            string _action = "Manager Reviewed";

            SourceCode.Hosting.Client.BaseAPI.SCConnectionStringBuilder connectionString = new SourceCode.Hosting.Client.BaseAPI.SCConnectionStringBuilder();

            connectionString.Authenticate = true;

            connectionString.Host = "localhost";

            connectionString.Integrated = true;

            connectionString.IsPrimaryLogin = true;

            connectionString.Port = 5252;

            connectionString.UserID = _user;

            connectionString.WindowsDomain = _domain;

            connectionString.Password = _password;

            connectionString.SecurityLabelName = "K2"; //the default label

            // open a K2 connection

            Connection connection = new Connection();

            connection.Open(_serverName, connectionString.ToString());

            try

            {

                // open the worklist item

                connection.OpenWorklistItem(_wfSerialNumber);

                worklistItem.Actionssaction].Execute();

            }

            catch (Exception ex)

            {

                // write error to console

                //Console.WriteLine("Error: " + ex.Message);

                //Console.ReadLine();

                throw();

            }

            finally

            {

                // close the connection

                connection.Close();

            }

        }