MySQL IF NOT EXISTS

In MySQL, how do I do an insert based upon whether or not a row already exists in a table? The SQL below doesn't seem to work, so I'm looking for another way to accomplish this. Below is all of the code that I have so far (I'm in the process of converting this from MS SQL). I'm sure there are several things wrong with it, but I am specifically looking for the if not exists syntax. If anyone would like to take a shot at making the whole thing work, then I won't complain. :)

CREATE PROCEDURE INTRA_UpdateEmployee (
in_Action varchar(10),
in_ID int,
in_FirstName varchar(255),
in_LastName varchar(50),
in_Company int,
in_Department int,
in_Title varchar(255),
in_Supervisor varchar(1),
in_EmailAddress varchar(255),
in_OfficePhone varchar(15),
in_HomePhone varchar(15),
in_CellPhone varchar(15),
in_FaxNumber varchar(15),
in_IMName varchar(50)
)

sp:BEGIN

IF in_Action = 'insert'
      IF NOT EXISTS (Select ID From employees Where ID = in_ID)
      THEN
            Insert Into employees (ID, Name, Password, InActive, Extension)
            Values (in_Action, in_ID, in_FirstName, in_LastName, in_Company, in_Department, in_Title, in_Supervisor, in_EmailAddress, in_OfficePhone, in_HomePhone, in_CellPhone, in_FaxNumber, in_IMName);

            Select 'Insert Successful.' as Message;
            leave sp;
      ELSE
            Select 'There is already a user with this name.' as Message;
            leave sp;
      END IF
END
ELSEIF in_Action = 'update'

      IF EXISTS (Select ID From Users Where ID = in_ID)

            Update employees
            Set firstname = in_FirstName,
                  lastname = in_LastName,
                  company = in_Company,
                  department in_Department,
                  title = in_Title,
                  supervisor = in_Supervisor,
                  emailaddress = in_EmailAddress,
                  officephone = in_OfficePhone,
                  homephone = in_HomePhone,
                  cellphone = in_CellPhone,
                  faxnumber = in_FaxNumber,
                  imname = in_IMName                  
            Where ID = in_ID;

            Select 'Update Successful.' as Message;
            leave sp;
      END IF

ELSE
      Select 'Invalid Action.' as Message
      leave sp;
END IF

END