Adding items to another Modules Admin form I have a module that I need to add a few items to, I was told that using hook_form_alter was
the best way to do this, but have been completely unable to get anything to work. Maybe some
one could help me out on it a bit....

The PURPOSE of this enhancement is to add text and links to a login modal, but that's unimportant
at the minute.

The code that I have in my module atm is just to show the form I'm trying to alter

&lt;?php
/**
 * Implements hook_menu_alter().
 */

function [modulename]_enhancements_menu_alter(&amp;$items) {
  $form['modal_window'] = array(
    '#type' =&gt; 'fieldset',
    '#title' =&gt; t('Modal text and links'),
    '#collapsible' =&gt; TRUE,
    '#collapsed' =&gt; 0,
    '#weight' =&gt; -399,
  );

    $form['modal_window']['login'] = array(
      '#type' =&gt; 'text_format',
      '#format' =&gt; 'plain_text',
      '#title' =&gt; t('Login text'),
      '#default_value' =&gt; variable_get('[modulename]_login_text', '[Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis a arcu et sem pharetra consectetur. Proin eget lectus neque, quis suscipit lectus. Nullam dictum, massa at sodales porttitor, mi nisi fermentum magna, eu suscipit dui. ]'),
    );

    $form['modal_window']['registration'] = array(
      '#type' =&gt; 'text_format',
      '#format' =&gt; 'plain_text',
      '#title' =&gt; t('Registration text'),
      '#default_value' =&gt; variable_get('[modulename]_registration_text', '[Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis a arcu et sem pharetra consectetur. Proin eget lectus neque, quis suscipit lectus. Nullam dictum, massa at sodales porttitor, mi nisi fermentum magna, eu suscipit dui.]'),
    );
}
?&gt;


The module has something like this going on with it
&lt;?php
/**
 * Implements hook_menu().
 */
function [modulename]_menu() {
  $items['admin/config/services/[modulename]'] = array(
    'type' =&gt; MENU_NORMAL_ITEM,
    'title' =&gt; '[modulename] Settings',
    'access arguments' =&gt; array('administer [modulename]'),
    'page callback' =&gt; 'drupal_get_form',
    'page arguments' =&gt; array('[modulename]_admin_form'),
    'description' =&gt; '[modulename] Configuration.',
    'file' =&gt; '[modulename].admin.inc',
  );
?&gt;

and here's the [modulename].admin.inc
&lt;?php
/**
 * Form builder for the [modulename] administration form.
 */
function [modulename]_admin_form($form, &amp;$form_state) {
  global $user, $base_path;
  include_once DRUPAL_ROOT . '/' . 'modules/user/user.pages.inc';
  module_load_include('inc', '[modulename]');
  $capabilities = [modulename]_get_capabilities();
  $form = array();

   $form['[modulename]_debug'] = array(
    '#type' =&gt; 'checkbox',
    '#title' =&gt; t('[modulename] debuging'),
    '#description' =&gt; t('Write errors to watchdog'),
    '#default_value'=&gt; variable_get('[modulename]_debug', 0),
    '#collapsible' =&gt; TRUE,
    '#collapsed' =&gt; TRUE,
    '#weight' =&gt; -401,
  );
?&gt;
For the sake of clarity, let's say that your custom module is named CUSTOMMODULE, and the one you want to alter is ALTERTHIS.

You can use hook_form_FORM_ID_alter() like this:

/**
 * Implements hook_form_FORM_ID_alter() for ALTERTHIS_admin_form().
 */
function CUSTOMMODULE_form_ALTERTHIS_admin_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // Alter the $form here.
}