Hi,
I assume you are using the vs2010 silverlight business application template. If you are not then the following might be irrelevant.
There is an article about this on the msdn website:
http://msdn.microsoft.com/en-us/library/ee796239(v=vs.91).aspx[
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Basically just after this line in MainPage.xaml
<HyperlinkButton x:Name="Link2" ... />
add a new button:
<Rectangle x:Name="Divider3" Style="{StaticResource DividerStyle}"/>
<HyperlinkButton x:Name="Link3" Content="Order List" Style="{StaticResource LinkStyle}" NavigateUri="/OrderList" TargetName="ContentFrame"/>
Hope that is what you are after.
Valery.