1. First of all, get this plugin (Allows you to have PHP in templates, which will help you out to get this custom shit).
2. Install & activate it.
3. Go to "ACP > Templates & Style > choose the template you are currently using > Member Templates > open and edit: member_profile_online
change all the code inside to this:
Sorry, but you need at least 25 posts and 5 hours online time to unlock hidden content.
Don't forget to change "UIDHERE" to your UID.