It's on the "Write" tab in the backoffice. Just below where you see Preview, Save, Spellcheck, etc. It's a little box which you can put a checkmark in. You should see a date and time already filled into the little boxes, as well.

Take another look, and if you still don't have it then we can figure out what to do next.

Check your user level. It affects a few permissions such as the ability to upload files / pictures and the ability to modify your timestamp.

You will probably need to log in as Admin to change the level of your user account.