When Ashfall is murdered and Sunstar tells you to go to Bushstep. Go to Bushstep (The cat all the way left of the camp, next to the med cat) and click on him (I'm pretty sure its a him, but I get genders wrong). He'll let you in that den just the one time (Unless you're blind) and you'll be able to get "detective equipment" including the map.
It'll appear in the top right corner.
If you're blind, when you become an apprentice on the medicine cat path, go to Bushstep and he'll make you run an errand for herbs. You'll get the map then.