Dynamic forms in tapestry are a bit complicated. Tapestry passes a t:formdata request parameter which contains the serialized form entities. This is then used serverside in the POST to re-hydrate initial form state. This must be kept up-to-date with what the client sees.
If you want to add dynamic content to a form via ajax, you will need to use the FormInjector. You might want to take a look at the source code for the AjaxFormLoop to see an example.
If you want to render hidden form fragments and make them visible based on clientside logic, you can use the FormFragment