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:
Click Settings, click Customizations, and then click Customize the System
Expand Entities, and then expand Lead
Click Forms and then open the "Lead" form
In the ribbon area of the form editor, click Navigation
Select Documents from the Relationship Explorer, and drag this entry to the desired location in the left navigation area of the form
Click Save and then click Publish
Repeat steps 2 through 6 for the Opportunity entity