Hiding Menu Items with Role

I have been trying to figure this out for a couple days, but am getting really confused on the many different posts/articles I have read.
I am trying to hide certain menu items based on the users role.
I have a Menu and a SiteMapDataSource on a master page:
<asp:Menu ID="NavigationMenu" runat="server" CssClass="menu"
  EnableViewState="false" IncludeStyleBlock="false" Orientation="Horizontal"
                DataSourceID="SiteMapDataSource1">
                <Items>
                    <asp:MenuItem NavigateUrl="~/Homepage.aspx" Text="Home"></asp:MenuItem>
 &n ...