'jestem na tak'
Do you know contest shows such as Idol, X Factor, Mam Talent, You can dance?
So it's famous slogan which appeared for the first time in such shows and step into colloquial language.
A jury member says:
Jestem na tak when he/she approve a candidate for the further contest.
Jestem na nie when he/she doesn't.
However expression used in Makowiecki's song has nothing to do with the latter. He sings that he sends a song "for no" as gift for those who are sad/depressed. You should read this as if he sends a song with a negative energy, a song which is not cheery and happy but pessimistic (therefore probably thruthful, honest... as the one line says: there one can sing what he wants).
Bear in mind that I've never heard that song, I've just checked the lyrics for you.
step into colloquial language.
of course when used in colloquial language it mean
I support (this idea)