Select the src folder and
    then choose  →  →  → . Create a new data model, call it
    example.data. On the last page of the wizard, select
    
Model
 as model object. 
Next, populate this very model as following. Please note that in the case of attributes you have to define a type as well (i.e. String), not just a name.
Again, to avoid any typos here is the XMI for
    example.data:
<?xml version="1.0" encoding="UTF-8"?>
<data:DataModel
  xmi:version="2.0"
  xmlns:xmi="http://www.omg.org/XMI"
  xmlns:data="http://www.xpand.org/xpand.demo.emf.datamodel">
  <entity name="Person">
    <attribute name="name" type="String"/>
    <reference name="cars" toMany="true" target="//@entity.1"/>
  </entity>
  <entity name="Vehicle">
    <attribute name="plate" type="String"/>
  </entity>
</data:DataModel>