I finally found the textbox (second one below). It was a matter of setting the Type="Edit".
When I was looking at the web page mentioned, I was first looking only at "children" thinking that I would see a textbox there.
<Control Id="Description2" Type="Text" X="135" Y="140" Width="220" Height="20" Transparent="yes"
NoPrefix="yes" Text="Full path to settingsFile:" />
<Control Id="UserSettingsFileName" Type="Edit"
X="140" Y="150" Width="160" Height="80" Property="SettingsFilename"
Text="C:\Path\SettingsFileGenerator.xml">
The initial value of the edit/box was not set to the text I specified. Any ideas on that? Do I have to set the property value outside of the control?
But if there is an reasonably easy-to-use "browse"/file-picker, I'd like to know about that too.