How do I enable a menu link only when the user has created a node?
Here are the steps

For example If you want give permissions for authenticated users to create 'Basic page'

Go to Home » Administration » People (admin/people/permissions) and check the checkbox

Basic page: Create new content and Administer menus and menu items for AUTHENTICATED USER 

That's it now authenticated  user can create basic page (node/add/page) with menu item. 

If you want to generate URL/path aliases automatically, use the Pathauto module 


  The Pathauto module automatically generates URL/path aliases for various kinds of content (nodes, taxonomy terms, users) without requiring the user to manually specify the path alias. This allows you to have URL aliases like /category/my-node-title instead of /node/123. The aliases are based upon a "pattern" system that uses tokens which the administrator can change.