- Go to your Facebook Page.
- Directly below your Page's Profile photo, click on "Edit Page".
- In your browser's URL, note the number that comes AFTER 'edit/?id=' That number is your Page ID.
- Go to this URL, but change REPLACEME to your Page ID from step 3: http://www.facebook.com/pages/edit/app_settings.php?id=REPLACEME&aid=2305272732
- Click the setting for "Allow All Fans to Tag Photos"
- Save your settings.
(Source: Nick Coleman on http://www.facebook.com/topic.php?uid=10381469571&topic=3794#topic_top)