Thanks again, Elena -
The code you posted was close, but it gave me the clue on what was intended. The Pages.g.cs file is generated and includes all the elements in the repository as properties. The WPF test step includes an instance of this class via the "Applications" property. The property itself appears to be an instance of XamlFindExpression, which needs to be passed to the Find method, like so:
var expr = Applications.AppName.WindowName.Expressions.RichTextBoxElementName;
var rtb = ActiveApplication.MainWindow.Find.ByExpression(expr);
Note: one problem using the "Applications" instance is that with a large number of elements, Intellisense in the code editor appears to fail. To find the names of the generated properties AppName and WindowName, I needed to load the compiled assembly in a tool like dotPeek and reflect on the members.