I am using firefox and i cannot fill the field saying it is from an other author it just does not appear.
It is a known bug. Unfortunately, it is a low-priority high-level-of-effort fix.
In the meantime, (after verifying the author has explicitly licensed the works with an OGA-accepted license) mark it "yes" and make a clear note of who the author actually is in the description including a link to the source page or author's website.
See also: http://opengameart.org/forumtopic/uploading-other-authors-works
--Medicine Storm