If it's an old traditional song (say, over 100 years old), then you can list that information the following way:
For a Public Domain track you can enter "Traditional" in the first name field then "Song" in the last name field.
In the publisher field when asked "Does this track have a publisher" select YES. Then list the publisher as Public Domain.
DO NOT put N/A or none in those fields.
If it's NOT an old traditional song, but you just don't know who wrote it, you need to do more research! You're not supposed to be recording and releasing someone else's song without paying the copyright owner of that song anyway, so if you don't know who it is, you're probably breaking the copyright law and we can't be putting that out into the lawyer-infested waters of digital distribution, until you do things correctly.