The ability to delete your posts should have been available from the start. If it is still not, let me know as that would be a bug.
It wasn't possible before, but then it seems it got changed after your post so it was actually possible again - however at the time of Writing this it is no longer possible again.
Edit: I'll leave it as it was above because it was technically true at the time, it seems it's possible to delete messages again. However what I wrote below is still true:
Also for some strange Reason I noticed a couple of minutes ago the function for Listing and Searching for Members disappeared as an alternative at the very top, normally it could be reached at http://silkroad5v7dywlc.onion/index.php?action=mlist but going there now it's just treated as going straight to the Index Page.