Hi,
You have to go in HikaShop.
Click on the menu Display > Custom fields.
Click on "New" in the toolbar.
Fill the field name in the "label" field.
Select the kind of custom field in the "Table" field. (product, if you fill the value. item, if the customer must fill the value)
Select the type of the field in "Field type" to display text field, radio buttons, ...
Enable the option "Frontend" if you want to display it for the customers.
Save.
If you have selected "product" and not "item":
Then edit a product (still in backend), and you should see the newly created custom field displayed, just fill this field and save.
It is now displayed on frontend.
If you have selected "item":
The custom field is already displayed in frontend.