Hi,
The "item" custom field is not editable in the "product page" but in the "order page".
An "item" is a product in the "cart / order", so you can access to the "item custom fields" when a product is in the "cart/order".
If you want to create a field that can be set in the "product page" in the backend, you have to create a "product custom field".
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.