Hi Ninama,
Thanks for letting us know about this. If you are creating a "fixed price" or "store item" format listing, there should be a box labeled "Accept Offers from Buyers" that you can check in order to use the "Best Offer" feature. If you are not seeing that option, we would need to look into your case further.
If this box is still not showing up for you, I recommend contacting our customer support team using the web form on the following page of our site:
http://www.auctiva.com/help/requesthelp.aspx-Mike