By default when you open a menu item based on your application's sitemap, it opens in the current window. To have items open in a new window, it's just a simple two step process. First add the "target" property to the sitemapnode you want to be opened in a new window:
<siteMapNode url="http://gemini/Reports/Pages/Folder.aspx?ItemPath=/DMG" title="Reports" description="Reports" target="_blank"/>
Next add the following code to your master page (or whatever page your menu resides on):
Protected Sub mnDefault_MenuItemDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.MenuEventArgs) Handles mnDefault.MenuItemDataBound
e.Item.Target = e.Item.DataItem(
"target")
End Sub