Everybody knows, now chatter widget is available to collaborate with the peoples or a group of people in your company. To enable the chatter widget to the users you should have "Customize Application" permission, if you have this permission follow the below steps.
Your Name-->Setup-->Customize-->Chatter-->Chat Setting
Now you are on the page to enable the chatter widget.You can see there is a two option
1. Chat Setting.
2. Visualforce Setting.