could any one explain even though i wrote standard controlller 
 <apex:page standardController="japan__c" recordSetVar="jap">
    <apex:dataTable value="{!jap}" var="a">
        <apex:column value="{!a.name}" />
    </apex:dataTable>
</apex:page>

i am unable to see Visualforce in page layout. do i need enable anything in specific .