Imovie-adding music to a project

I am a first time iMovies user and I have taught myself through the majority of the process. I've uploaded all my clips to my first project and am ready to add music and when I try it tells me my "computer is not authorized to play this content." I have purchased & downloaded the particular song. When it tells me to authorize my computer and I enter the iTunes password to do so it tells me my computer already is authorized. I've googled this issue all over and the only real solution I have found was someone suggesting I burn the song to a CD and then re-upload it to the computer. Problem with that is I have a MAC and no CD-rom. Any help at all would be greatly appreciated! Thanks in advance!