
From your Dashboard, click on the name of your Tumblr blog at the top of that page. (Even if that name is “Untitled” this should still work.) Now from the right-hand sidebar at your blog page, click the “Settings” box.

Then from the “Settings” page, find and click the checkbox beside “Let people ask questions”.
That checkbox is circled in red in the example below.

Scroll to the end of the “Settings” page and click “Save Preferences”.
That’s it! :-)
An apology: When I began returning confirmation letters for the S-Tumblr Directory, I didn’t realize that many applicants had not yet set up their “Ask” boxes. I returned early confirmations via “Ask”, and now I fear those early confirmations were lost. Please check the Directory if you think I may have missed your request.
http://s-tumblr-directory.tumblr.com/
Please re-submit your request if I have missed you. Thank you for your patience.
—Chris