1. From Setup, click Manage Users | Users.
2. Click Edit next to a user's name.
3. Select Service Cloud User.
4. Click Save.
But, until Winter '14 release, we cannot run report on which users has this license enabled. It is too difficult if have to check each users. The workaround is to use tool with SOQL, such as Data Loader.
The field on the user object which holds the "Service Cloud User" checkbox value is "USERPERMISSIONSSUPPORTUSER". Using the data loader, we can export a list of users with service cloud user checkbox enabled by adding a filter USERPERMISSIONSSUPPORTUSER=TRUE
Select Id, Username from User where USERPERMISSIONSSUPPORTUSER = True
This field does not show up when you are using the Data Export Services. So tool with SOQL call is the only workaround as of now.
Assigning the Service Cloud Feature License to Users
How to get a list of service cloud console users?
Make the Service Cloud User field available for User List Views