I've solved this issue as follow:
1. In Settings > Control Panels > Wysiwyg
1.1. Pluging WYSIWYG hast to be checked.
1.2. Check the option "Use Wiki syntax in WYSIWYG"
2. In Settings > Permissions > Wiki
2.1. for the non-admin user assign the permission for "Can view source of wiki pages (tiki_p_wiki_view_source)" (thanks Rick for your advice)