Trying to link aspnet_users to my table with userID as foreign key

Perhaps this is a simpler VS9 question, but I want to link a SQLServer table with additional user data to the standard SQLServer membership table using the GUID userID as a foreign key.  I go into design mode of my table, right-click on relationships, and click add in the dialog.  but the left hand column shows me only the tables in my database.  How do I get it to show the tables in the ASPNETDB.MDF database?  I went through the tutorial (http://www.asp.net/learn/security/tutorial-08-cs.aspx), but it seems to magically appear in there without explanation.
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