An illustration of how this is done can be seen at the Salesforce website here:
https://help.salesforce.com/apex/HTViewHelpDoc?id=content_initialsetup.htm&language=en_US.

Here are the instructions for how to do it:

Enable Salesforce CRM:
1. Go to Customize | Salesforce CRM Content | Settings and select Enable “Salesforce CRM Content”
   
2. On the right side, click Edit, then select “Enable Salesforce CRM Content”:
   

The profile of the user doing the sync should have the following permissions enabled:

• Manage Salesforce CRM Content
• Create Libraries
• Manage Content Permissions
• Manage Content Properties
• Manage Content Types and
• Deliver Uploaded Files and Personal Content
3. Go to users
   
4. Click the profile of the user (If this user is an admin, then the profile name should be “System Administrator”
5. Scroll down under “Administrative Permissions” to find and enable “Manage Salesforce CRM Content”:
   
6. Scroll down under “General User Permissions” to find and enable “Create Libraries,” “Manage Content Permissions,” “Manage Content Properties,” “Manage Content Types,” and “Deliver Uploaded Files and Personal Content”:
   

Check that everything is ok in Salesforce:

7. Go to your Salesforce “Libraries” tab. Create a new library for the files from your Box folder:
   
   
   
8. Then go to “Files” tab. The new library should be on the left side:
   

After the library has been created, you can create the sync pair.

NOTE: You must have a Salesforce Business/Enterprise Plan to sync using cloudHQ.

9. Sync Salesforce by clicking this wizard

https://www.cloudHQ.net/sync/salesforce/box

10. Here are instructions for how to set it up:

https://support.cloudhq.net/how-to-sync-salesforce-and-box/