You need to stumble into the "User Control Panel" part of the site. The only way I've found to get into that area is by clicking on the "Good evening , you have # messages..." at the very top of the site. The "# messages" is a link that drops you into a portion of the User CP. From there you can get to your posts under the View -> Posts tab on the left, and "View my last # posts".
I think you should be able to click on this link: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?action=mycenter and it should work for you.
Not exactly intuitive. 