Hi – when you’re logged in as an admin, you should be able to do this from the front end as well as in admin. There should be a little bar of links at the top of each post like the one in the image on the right here: http://www.ilovecolors.com.ar/add-private-content-to-bbpress-topics-and-replies/ (the plugin has nothing to do with this issue, it’s just the best image I could find).
If you don’t see that, make sure your bbPress role is set to Keymaster. And if that is right, make sure another plugin or your theme isn’t causing them not to appear by disabling all plugins then re-enabling them one by one and switching to the Twenty Fourteen theme.
Thank you. Excellent answer. But let me extend the question, please. Is there no way to moderate the forum in the sense of reserving the right to approve submissions before they are posted?