Hi,
You need to create a custom field of the table "order", and not of the table "entry".
Once done, you need to create a new instance of the plugin via the "new" button of the System>Plugins menu of HikaShop, and not via the Joomla plugins manager.
And in there, you'll have to specify the column name of the custom field.
The plugin is only there to add the fee to the cart. But the display of the input field is done by the display settings of the custom field itself and the "fields" view of your checkout workflow. So you'll have to make sure that you still have the "fields" view in your checkout workflow in the Hikashop configuration and that you activate the display settings of the custom field so that it appears on the checkout.