I just did a quick test. I selected "Fixed Price" and then a Category I know is approved for Best Offer. The "Accept offers from buyers" checkoff box appeared directly below the greyed-out Reserve Price box on the lister.
Just to double-check, I removed the Category and the checkoff box disappeared.
Are you using IE or Firefox?
Suggestion: Try what I just tried using New Listing. If that doesn't work, try clearing cache and cookies for your browser.
Danno