If you want to use a audience field on a pagelayout you can use the example below in your aspx. 

...
<%@ Register tagprefix="FieldTypes" namespace="Microsoft.Office.Server.WebControls.FieldTypes" assembly="Microsoft.Office.Server, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
...
<asp:Content contentplaceholderid="PlaceHolderMain" runat="server">
...
    <PublishingWebControls:editmodepanel runat="server" id="editmodepanel">
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                    <FieldTypes:SPFieldTargetToControl runat="server" FieldName="Audience" AudienceTypes="GlobalAudience" />
...
    </PublishingWebControls:editmodepanel>
...
</asp:content>