First, it's enabled in the
ACP via a master switch:
ACP>Configuration>unread posts
It's the first one at the top where you decide to activate it.
Then if you activate the radio button "for users", then it can be turned off and on in the user's profile.
Profile>preferences> now it's at the bottom of that page where you select your settings.
I'm trying by the way to get it just to show the number by the "New" link in the header bar........I'm not having any luck.