IMHO , that's impossible to add the buttom without changing to the forum soft. Its code has to be chagned what is not easy to do. If you want to see your latest post just move the cursor over your nick below your avatar and click the message number in the pop-up tab. You will be moved to the page with all your posts listed from the oldest to the latest one as the first ( on the top ). Then click the post on the list.
Also you may click your avatar thumbnail at the top bar on the right. Then click the number of messages there.