Users cannot delete their own posts. If you need a post of yours deleted because it's a duplicate or for some other reason, use the report this post to moderator function on that post and simply explain why you need/want it deleted.
As to why and any further information, see either of these threads:
<li>
http://tfproject.org/tfp/showthread....ht=delete+post
<li>
http://tfproject.org/tfp/showthread....ht=delete+post