The sample provided sample_places.xml for select box capability, does not contain the 'displayname' attribute.
The way to include it is through an additional attribute in the .xml file called 'displayName'. Please note the capital 'N'.
Here is an example of the .xml file sample_places.xml provided with IM but modified including 'displayName' field:
<places name="places" displayName="places-display-name">
<country name="Australia" displayName="AUS">
<state name="Victoria">
<city name="Melbourne" displayName="MEL"/>
<city name="Geelong" displayName="GEE"/>
<city name="Ballarat" displayName="BAL"/>
</state>
<state name="NSW">
<city name="Sydney" displayName="SYD"/>
<city name="Newcastle" displayName="NCS"/>
<city name="Wodonga" displayName="WDG"/>
</state>
<state name="Queensland" displayName="QUE">
<city name="Brisbane" displayName="BRI"/>
<city name="Cairns" displayName="CAI"/>
<city name="Townsville" displayName="TWN"/>
</state>
</country>
<country name="United States" displayName="USA">>
<state name="New York">
<city name="New York City" displayName="NY"/>
<city name="Albany" displayName="ALB"/>
<city name="Ithaca" displayName="ITC"/>
</state>
<state name="Massachusetts" displayName="MCH">
<city name="Boston" displayName="BST"/>
<city name="Cambridge" displayName="CAM"/>
<city name="Framingham" displayName="FRM"/>
</state>
<state name="California">
<city name="Los Angeles" displayName="LA"/>
<city name="Sacramento" displayName="SA"/>
<city name="San Francisco" displayName="SF"/>
</state>
</country>
<country name="Italy" displayName="ITA">>
<state name="Lazio" displayName="LZ">
<city name="Rome" displayName="RM"/>
<city name="Viterbo" displayName="VB"/>
<city name="Latina" displayName="LAT"/>
</state>
<state name="Tuscany" displayName="TUS">
<city name="Florence" displayName="FLO"/>
<city name="Pisa" displayName="PIS"/>
<city name="Siena" displayName="SIE"/>
</state>
<state name="Campania">
<city name="Naples" displayName="NAP"/>
<city name="Amalfi" displayName="AMA"/>
<city name="Sorrento" displayName="SOR"/>
</state>
</country>
</places>