Hi, Kendall,
I found this thread, from September - https://productforums.google.com/forum/#!topic/youtube/etKZt3ToLLU
When using the YouTube music library, I download the track to my hard drive, then add it as I create my video. However, I don't create my vids online, which I guess is a very different process.
Good luck getting this one figured out; it seems to be an ongoing problem -- especially for those using a Mac.