implementation of hook_menu() is not working
There are 2 things going on: you've not defined the permissions on the menu &amp; you've not defined the page callback for when the 'instrument' path is accessed. Without this, Drupal doesn't know when or what to display for your 'instrument' menu item. 

Doing something like this:

  $items['instrument'] = array(
    'title' =&gt; 'Instrument',
    'type' =&gt; MENU_NORMAL_ITEM,
    'menu_name' =&gt; 'main-menu',
    //@TODO: define a function called mymodule_instrument_page_callback 
    // that will render the page for the 'instrument' path.
    'page callback' =&gt; 'mymodule_instrument_page_callback',
    // Setting 'access arguments' =&gt; array('view content') 
    // will allow anyone with the 'view content' permission to view the menu item.
    //'access callback' can be used for dynamically determining access
    'access arguments' =&gt; array('view content'),
  );


Will make your menu item available. See the hook_menu() docs for information on page &amp; access callbacks.