You need to add the id attribute for the HBox, TextInputs and ComboBox dynamically and then you can access them in ActionScript.
The 'id' attribute does special compile-time processing, and setting the 'id' at runtime is not the same as setting it at compile time. When you write
the compiler autogenerates code similar to
public var b1:Button = new Button();
so that you can refer to the Button as b1. This isn't possible at runtime because you can't create a new public var in a class at runtime.
The best solution is to declare an Array or Vector to hold your dynamically-created components:
public var dynaComps:Vector.<UICompoment> = new Vector.<UIComponent>();
When you create a component, add it to the vector like this:
var ti:TextInput = new TextInput();
You can then refer to the ith dynamically created component as dynaComp[i].
Adobe Flex SDK Team
Thanks for correcting me, your solution is clear.