Hovering over the <Insert> button brings up a secondary menu that allows you to choose whether to insert a release or iteration (if you just click Insert it defaults to inserting a release). In either case, it will insert the new release / iteration above the currently selected item – i.e. the one whose check-box has been selected, at the same level in the hierarchy. If you want to insert a release/iteration below a summary item, you need to insert it first, then indent it with the <Indent> button. If you insert a release without first selecting an existing release from the list, the new release will simply be inserted at the end of the list.
Once the new release has been inserted, the item is switched to “Edit” mode so that you can change the default name, active flag, version number and creator.