How to use Hierarchial Select Module for select field's of webform module?
Seems like webform doesn't support Hierarchical Select module...

You can use Webform Conditional module to create dependent fields...

Project page says


  This modules enhances the existing Conditional Rules of the Webform
  Module. It adds the ability to make components dependent on select
  components(i.e. listboxes, radio buttons, and checkboxes) that appear
  on the same page.