Top N filter for an MDX DataSet in Reporting Services

Reporting Services: Can you please tell me how do I use the filter option from the DataSource tab in order to retrieve the top N records from the dataset.

I tried several ways, see below, none of them work, I keep getting the error that "cannot evaluate filterx expression/filter values".

<DataSets>
    <DataSet Name="FoodMart_2000">
      <Fields>
        <Field Name="Category_Category_Description">
          <DataField>[Category].[Category Description].[MEMBER_CAPTION]</DataField>
          <rd:TypeName>System.String</rd:TypeName>
        </Field>
        <Field Name="Measures_Amount">
          <DataField>[Measures].[Amount]</DataField>
        </Field>
      </Fields>
      <Filters>
        <Filter>
          <FilterExpression>=RowNumber()</FilterExpression>
          <Operator>TopN</Operator>
          <FilterValues>
            <FilterValue>2</FilterValue>
          </FilterValues>
        </Filter>
      </Filters>
      <Query>
        <DataSourceName>FoodMart 2000</DataSourceName>
        <CommandText>select
{[Measures].[Amount]} on columns,
{[Category].[Category Description].Members} on rows
FROM
Budget</CommandText>
      </Query>
    </DataSet>

or with:

       <Filter>
          <FilterExpression>=Fields!Category_Category_Description.Value</FilterExpression>
          <Operator>TopN</Operator>
          <FilterValues>
            <FilterValue>2</FilterValue>
          </FilterValues>
        </Filter>