Error in XML Schema Part 0: Primer

I am trying to understand how to use namespaces in XML Schemas.  I tried to
validate the po1.xml example using the po1.xsd given in the examples in
section 3.1 and received errors (using XML Spy 4.1).  The error I received
is "Unable to locate a reference to a supported schema kind...within this
document instance".

The XML Schema Part 0: Primer also states for this example:

   "The namespace is the same as the target namespace of the schema in po1.xsd, and so a processor of the instance document will know to look in that
   schema for the declarations of purchaseOrder and comment. "

However, there is no reference to po1.xsd in po1.xml, so how will it know
to 'look in that schema'?

     Janice Champion