Hi Colin,
You will need to edit the layout.master file of your chosen skin. For the artisteer-24verticalmenu3 skin specifically, you need to add the following directly above the <portal:SiteMenu control on line 80.
<asp:Panel ID="divAlt1" runat="server" CssClass="altcontent1">
<asp:ContentPlaceHolder ID="altContent1" runat="server"></asp:ContentPlaceHolder>
</asp:Panel>
Once you save this, you will have an additional content panel available named "Alternate Pane 1" to which you can add content.
HTH,
Joe D.