Rick,
There are a couple approaches to make this work. The easiest is to use FindControl and pass in the ID of the mastermenu on your master page. You'll need to cast it to the correct type. You may need to add a <%@ Reference VirtualPath="<.ascx file>" %> directive to your page for the user control in this case.
CType(Me.Master.FindControl(<id of the control), MasterMenu).Build(...)
Another thing you can do is expose the control as a property on your master page and then set the <%@ MasterType %> directive on your content page markup. Then you can access the control directly:
Me.Master.MyMasterMenu.Build(...)
--
Danny
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