Navigate to the menu by clicking the sub menu items (e.g. Main Menu > Sub Menu Items link - see image on the right)
Click Add New
Position will be filled in automatically
Type in a Title (this will be what is shown on the menu)
Type in a Description of the page / website the item links to (the description can be viewed on hover over)
The Image property can be used to have a little icon next to the menu item, click Browse and select an image to use from your computer
Type the URL
If linking to an existing page on your website type the File Name ONLY (go to Content Management > Pages to view the list of pages on your website and their file names - first column)
Use the Allow View By option to show or hide the menu item
Click Submit
Change Menu Position
In the list of menus use the Position Up and Position Down buttons to move the menu item
You can also type in a number in the Position column and click Change Position
Edit
Go to Actions > Edit on the right-hand side of the menu item you want to edit
Add New