1. Enable Content
   Setup | Customize | Salesforce Files | Settings | Salesforce CRM Content 
   • Select Enable Salesforce CRM Content.
2. Give users access to Content by assigning feature licenses in one of the following ways:
   To assign feature licenses to users automatically, click Setup | Customize | Salesforce Files | Salesforce CRM Content.  
   • Select Autoassign feature licenses to existing and new users.
3. To assign feature licenses to users manually, click Setup | Manage Users | Users and click Edit next to the user's name.
   •  Select the Salesforce CRM Content User checkbox and click Save.
4. Verify that the profile assigned to Content users has the Salesforce CRM Content user permissions enabled. The user permissions include “Manage Content, “Create Libraries,” “Manage Content Permissions,” “Manage Content Properties”, “Manage Content Types”, and “Deliver Uploaded Files and Personal Content.”

For More information See Force.com's: Setting Up Content