Following are the steps to add an external link to menu in Drupal:
1. In the navigation menu click Administer -> Site Building -> Menus
2. Choose the menu you would like to add the external link to
3. At the top click Add Item
4. Under Path enter the url of the site you would like to link to.
5. Under Menu link title enter the name that is to be shown on the menu.
6. You may enter a description for the menu item
7. Choose it’s parent
8. Under weight you can set where the menu item will appear in the menu. A smaller number will make it appear higher than a bigger number.
9. Click Save