Article ID: 2792854
After enabling SharePoint document management for the Lead and Opportunity entities, the "Documents" navigation link does not appear in the left navigation area of the form.
This is a known issue in the December 2012 Service Update and will be addressed in a future update.
As a workaround to the issue, the Documents navigation link can be manually added to the left navigation area on the new Lead and Opportunity forms. This is documented in the steps below:
(http://go.microsoft.com/fwlink/?LinkId=151500)for other considerations.