Just enable commenting in the K2 Settings. In there you can also add some protection for your comments form using a web service like Google's reCaptcha or Akismet. Set commenting enabled for everyone and then make sure the category that this item belongs to has comments enabled (3rd tab in the related K2 Category edit form, scroll down to the "item view" options).

If this K2 category is common to other K2 items for which you don't want comments to show, then you can only enable comments for that specific K2 item by flipping on the relevant option, again in the 3rd tab of the K2 item edit form, scroll down to the "item view" options.

In any case, there are 3 places where K2 is configured. The component's settings (mainly used for global settings for K2), the category and item edit forms (the latter can override the options set from the category) and finally the menu items (where any changes set there will be the last point to override any other previous option set in either the K2 Settings or a K2 category/item).