XSL transforms: How do I identify PropertyKey–PropertyValue pairs

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C#

XML

XSLT

A web service supplies me with XML in the form:

<HOUSEDATA>
  <PROPERTY>
    <PROPERTYKEY>CITY</PROPERTYKEY>
    <PROPERTYVALUE>BOSTON</PROPERTYVALUE>
  </PROPERTY>
  <PROPERTY>
    <PROPERTYKEY>STREET</PROPERTYKEY>
    <PROPERTYVALUE>CENTRAL ST. WEST</PROPERTYVALUE>
  </PROPERTY>
...
</HOUSEDATA>

I understand XML Nodes and elements.

How do I get XSL transforms to identify values by these
PropertyKey–PropertyValue pairs?

Any help is appreciated; especially help related to Altova Mapforce.

Thanks.

[Modified: added code formatting]