The site map depends on a hierarchy, with one single root not and multiple child nodes. However, you can instruct ASP.NET not to show that root node, so it only shows children. The SiteMapDataSource control has a ShowStartingNode property, which you can set to false to make it not show the root node.
Or, for a menu use:
<asp:Menu Orientation="Horizontal" StaticDisplayLevels="2"
This won't ignore the root node, but will treat it as though it were at the same level as it's children.