The Last edited thing at bottom of post

Have a new forum but cant seem to figure out how to add things like :

"Last edited by on Tue Aug 14, 2007 7:25 pm; edited 20 times in total"

everytime a user edits there post or i edit it, and help would be apreciated

Enter text where it asks you what you edited it for.

i cant see how this helps i am editing posts but im not getting this on the bottom of edited posts