Once logged into the administration area of your website, hover over the menu item "MENU" just under the header. Then click the menu you want to add an item to. Usually the menu named "mainmenu" is where you want to add this if it is the Main Menu of your site. Once the page has loaded click the "NEW" button towards the top right of the screen. Now you have to select the kind of menu item you want. This would be the following:

Menus -> Article Layout 

Once selected, you have to select the article you want to link to by clicking the select button in the right column under Parameters Basic. You will find a list with view option of the articles you have, the list only displays 20 per page, you can change this option right at the bottom to display them all if you cannot find the article you have written. Once you have selected the article click it and it should return you back to the options screen. Enter the title of the link under title, alias is what the page name is going to be for example if you enter about me the page will be able to be found at www.yoursite.co.uk/about-me.html . The parent items allows you to put the link under a section, for example if you want it under home,click home and it will place the menu item as a sub menu under menu item home. Published can turn the menu item on and off so you can remove it without deleting it. Access level is to restrict the page to people who are  registered, so they have to be a member to view the information. And under that you can control how the new page is opened, it can be a pop up or as one page to another. Once finished click save at the top right of the screen and you will be taken to the list of menu items.