The main menu is maintained outside of all the pages in your department/unit.
There are a couple of ways to work on your main menu:
The preferred way
The least error-prone way to add a page the main menu is to add it from the pages themselves.
So, to start, edit a page that you want in your menu.
At the bottom of the page you will see a section for "Menu settings".
If you check the "Provide a menu link" option, then you will see more fields about adding the page to the menu.
The "Parent item" field is the important one. It will show the hierarchy of the menu. If there is nothing in the menu yet, you have to just choose the top-level item (in this example, <SMART - Main Menu>).
If you save the page now, you will see that there is a menu with only one link in it - to the page you just added.
When you do this to another page, the "Parent item" field will now have more than one option. If you select the top-level item (<SMART - Main menu> in this case) then you get another top level menu item. If you select the other page, then it will live under the link and you will have create a drop-down into a sub-menu.
The Alternate way
Note that this is a great way to re-arrange an existing menu, or to fix small things like link text, but it is not the best way to add new pages.
The Edit Menu link
The Site Management menu option in the Shortcuts menu at the top of the screen will take you to a page with an Edit menu link.
Click on Site Management and you will see a screen like this:
Editing your main menu
In the bottom right corner there is a box for "Configuration" and that contains the "Edit menu" link. Select that.
You will now see a listing of all the menus that you have access to.
Select "Edit menu" button on the right side for the menu you wish to edit.
If you have not started the menu, it will look like this:
Adding menu links from the menu screen
By selecting the "+Add link" button, you go to a screen for adding an individual menu link.
The page looks like this:
The menu link title will be the wording that shows up in the menu.
The link field lets you start typing the name of the page you want to link, and then select one of results. Be careful here, you might end up linking to pages that are not part of your department. This is why it is not the preferred way of adding new pages to the menu.
When to go to the menu screen
It is useful to go the menu screen if you wish to change the ordering of the menu, or you want to repair the link text. This is a fast way because of how the menu gets presented to you.
This page shows you the hierarchy of your menu, and allows you to edit individual links in case you need to fix them.