I have an extra field called "additional information" and another one "company information". how to enable TinyMCE on both?

Thanks

@Cecile
If you go to oc-includes/osclass/functions.php

You may find code that generates tinymce here.
You may add new selectors

  <script type="text/javascript">
    tinyMCE.init({
      selector: 'textarea[name^="description["]',
      width: "100%",

ie.
  <script type="text/javascript">
    tinyMCE.init({
      selector: 'textarea[name^="description["], textarea[name="customtextarea"]',
      width: "100%",