Cannot hide menu option by roles.

 I've been through many posts on this topic and I still can't get it to work.  I'm trying to hide the "Admin" menu option until a user with an "Administrators" account logs in.   With the code below the menu option is not hidden.   The strange thing is that when I remove the url attribute in the siteMapNode tag for the Admin menu option it displays as expected:  not there at first and then I'll log in with the appropriate account and it displays. (Of course, it doesn't actually work as there is no associated url. )
Thanks is advance!
 
<?xml version="1.0" encoding="utf-8" ?> <siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" > <siteMapNode roles="*" > <siteMapNode title="Home" url="~/Default.aspx" roles="*" /& ...