Dear Norbert,

To download files of the atlas just go to the Download link (left bar of https://www.nitrc.org/projects/iit/ ).
The Contents_of_IIT_atlas file describes all the files of the atlas. The HOWTO files give step by step instructions on how to accomplish different tasks. If you need more guidance let us know what you are trying to achieve and we can help you further.

Regards,
Konstantinos