When you create a body from a template, the creation function returns an ID. This is the same as any DBPro ID, except that the plugin chooses a free ID automatically for you. You can use this ID to reference the body from then on, until it is destroyed.
edit:
If you mean how to get the body under the mouse, look at the 'contact listener' example on the wiki:
http://diggsey.heliohost.org/box2d/wiki/index.php?title=Sample_Contact_Listener
It shows you how to select an object when the user clicks, and uses a mouse joint to drag it around.
