How can I enable chatting functionality?

How can I enable chatting functionality?

We are using the Professional Edition of CRM.

In the post below : 
https://www.zoho.com/crm/help/miscellaneous/chatting-with-users.html#Activate_Chat_for_Users
it says that we can activate Chatting functionality inside the system via Setup/Users/Profile screen. Whereas I cannot see a checkbox to enable that functionality.

I've checked the license details and saw that group chatting is included in our license.

How can I enable chatting for my users?

Regards,
Haluk