Normally, ID's such as those in your menu are used to provide a persistent page indication.
This basically means, when you are on the Home page, the home link will be highlighted. When you are on the About Us page, that page's link will be highlighted, etc.
This method also normally uses a unique ID for the body tag of each page, so the body tag of the Home page might look like this:
<body id="homepage">
The body tag for the About Us page may look like this:
<body id="aboutpage">
...and so on.
Do the body tags for your pages have ID's?
If not, then this menu code may have been reused from another project by the theme's devoloper, and it may not matter if the new menu link you create has an ID. However, if the body tags do have ID's, then make sure you haven't overlooked some styling in the style sheet that might look something like this (the body id's are based on the examples I made up above).
#homepage #home a, #aboutpage #about a, #servicespage #services a, #photospage #photos a, #contactpage #contact a {
some:styling;
some:styling;
}
There can be other variations on this. Maybe instead of "#homepage #home a" it has "body#homepage li#home a:link".
If you do see this sort of code, you should create unique ID names for your new link and the body tag of the page it points to. Then you need to add them to the selector of the style rule you found for them.