Welcome to the 405th, your one stop shop for all your Halo costuming needs.
You can find all the files you want on the Armory tab on the site. There it is divided into game, films, etc.
Pepakura can import 3D models (obj and stl files) and use them to be unfolded. Just go to "Open", and select the file you would like to import. Afterwards, you will have to unfold it manually.
But, it is a good rule of thumb to clean up the model for easy unfolding. Often times, there are floating vertices that can make the Pepakura part confusing.