How do I add a node add link ( for a different content type ) in the node add form (drupal 7)
The module your are looking for is References dialog.

Also, you can use contextual administration. It's a more general approach, that won't put the link exactly where you want, but is much more flexible on the other hand. It allows you to use the same rule set as Panels, such as "Selection rule: Node type", for example to node/add/[other-type]. Fits like a glove together with Panels.