Hello,
I'm trying to add my custom code to item page but nothing appear.
please, If any one can tell me in which page can I add my code

Hi,

the proper way to alter the file is by using template overrides for K2.

Read more here:

getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates

or here

getk2.org/documentation/k2class2012/865-k2-template-and-overrides

The file you need to edit is item.php

Best regards