Why Can’t I Add Users in my Contact list?

I get message saying can’t add user. Why is that?

Most probably the feature “Manage contacts lists” is disabled for your Instant Messaging network. Please, contact your IM network administrator and ask to allow this feature for your account.

Or the user is already added, but you cannot see it in your contacts list because he was added after you signed in. Please, log out and then login again.