First, create an article if you have not already.

• Go to Menus and then click on mainmenu, or the menu where you wish to have your link.
  

• Click New.
  

• Click on Articles and then Article Layout.
  

• Enter the Title (this will be the titled displayed in your menu).
  The alias will automatically be generated.
• Click the Select button and then the Article (from the list in the pop-up window).
  Remaining settings should be left as default.
• Click Save