If a song's music & lyrics were published before January 1, 1928, then the song is considered in the Public Domain in the US. Please visit
pdinfo.com to learn more about Public Domain works and see a list of songs that qualify. It's important to know that if a work falls within the Public Domain, only that exact version is not protected under US copyright law. Other people may have created arrangements of that work that ARE under copyright. Do your research, please!
If you have a public domain song then you can list that information the following way:
Songwriter
For a Public Domain track you can enter "Traditional" in the first name field then "Song" in the last name field.
Publisher
In the publisher field when asked "Does this track have a publisher" select YES. Then list the publisher as Public Domain.
DO NOT enter your name, “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.