If you want to add something to a menu, you need to implement an `onOpen` function, and do the necessary menu setup there.

Take a look at this example from [Google's documentation][1]:

    function onOpen() {
      // Add a custom menu to the spreadsheet.
      SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
          .createMenu('Custom Menu')
          .addItem('First item', 'menuItem1')
          .addToUi();
    }

This will add a menu _Custom Menu_ with a single item _First item_, which, when selected, will launch a function `menuItem1`, which you will have to implement:

    function menuItem1() {
      // Do whatever you like here
    }

Note that you will have to reload your spreadsheet in order to have `onOpen` run. So you won't see any effect by just writing the script and clicking save, you need to reload (<kbd>Cmd</kbd>-<kbd>R</kbd>) on the spreadsheet window for the new menu to appear.

A script error may prevent the menu from appearing. If you don't see the desired result, check the script editor's _View &rarr; Execution Transcript_ for any errors.
  [1]: https://developers.google.com/apps-script/guides/triggers/#onopen