try removing <div class="art-nav-inner" from layout.master and specify it in theme.skin as the container class for your menu like this in theme.skin:
<portal:mojoMenu runat="server" SkinID="SiteMenu"
RenderContainerCssClass="true"
ContainerCssClass="art-nav-inner"
RenderImages="false"
UseMenuTooltipForCustomCss="true"
RenderCustomClassOnLi="true"
RenderCustomClassOnAnchor="false"
RenderLiSelectedCss="false"
RenderAnchorSelectedCss="true"
UlCssClass="art-hmenu"
LiCssClassWithChildren=""
LiCssClassWithoutChildren=""
LiSelectedCssClassWithChildren=""
LiSelectedCssClassWithoutChildren=""
LiChildSelectedCssClass=""
LiParentSelectedCssClass=""
AnchorCssClass=""
AnchorSelectedCssClassWithChildren="active"
AnchorSelectedCssClassWithoutChildren="active"
AnchorChildSelectedCssClass=""
AnchorParentSelectedCssClass=""
InnerSpanMode="Artisteer"
/>
this is assuming that is the correct structure as seen in the index.html exported by Artisteer.
Hope that helps,
Joe