How to access DataTemplate in ResouceDictionary of UserControl

Question

I have a resource dictionary as follows defined in my UserControl:

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="../ParameterEditorResourceDictionary.xaml"></ResourceDictionary>
            <ResourceDictionary>
                 <DataTemplate x:Key="ParameterDefault">
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="("></TextBlock>
                            <ItemsControl ItemsSource="//I want to set from code">
                            //some more items here
                             </ItemsControl>
                        <TextBlock Text=")"></TextBlock>
                    </StackPanel>
                </DataTemplate>
            </ResourceDictionary>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>       
</UserControl.Resources>

Then I have a DataGrid in my UserControl which has a loaded event:

<cc:PEDataGrid AutoGenerateColumns="False"
               Loaded="CommonPEGrid_Loaded"
               >        
</cc:PEDataGrid>

In the loaded event code I have to access the DataTemplate and then traverse to ItemsControl of it and I want to set the ItemsSource property from code. But I am Unable to do it. My code is as follows:::

       DataGridTemplateColumn column = null;
       int i = 0;

       foreach (ParameterLoop obj in tempObj.ParametersAllLoops)
        {
            column = new DataGridTemplateColumn();
            column.Header = "Loop ( " + i.ToString() + " )";


            //not working
            DataTemplate dt = FindResource("ParameterDefault") as DataTemplate;
            Type t = dt.VisualTree.FirstChild.GetType(); //VisualTree always null


            //not working
            DataTemplate dt = Resources["ParameterDefault"] as DataTemplate;
            Type t = dt.VisualTree.FirstChild.GetType(); //VisualTree always null                     

            //I am not able to traverse the visual tree at all !!
            //Where am I going worng??


            column.CellTemplate = dt;
            dg.Columns.Add(column);
            i++;            
        }

I tried to google out, but I am not understanding the solutions given by many!! Please help me regarding this!! A sample illustration would be great.