I've just found how to solve it…The code -based on "http://www.deliciousdays.com/cforms-forum/troubleshooting/tutorial-select-box-selection-shows-hidden-fields/#p19866" for showing-up the textfield is the following:

————

$(document).ready(function() {
  $.viewMap = {
    '0' : $([]),
    'OTRO' : $('#id_of_the_textfied'),
  };
$.each($.viewMap, function() { this.hide(); });
  $('#id_of_the_radiobutton_other').change(function() {
    $.each($.viewMap, function() { this.hide(); });
    $.viewMap[$(this).val()].show();
  });
————

Now i'm trying to create a function that checks that if "other" is checked and "textarea" is empty returns an "pop-up error message" and if "other" is not checked and textarea is not empty returns other "pop-up error message".

Im beginning w/ my_cforms_action() but i think i'm not in the right direction. Any suggestions??

Thanks in advance.