Hi! I'm Eric, a Mac expert who's been paired with you through JustAnswer.com. I'm reviewing your question now, and will return w/ an answer soon!
Hi Lyle! This problem can often be fixed by consolidating your iTunes media and reorganizing your library. To do this:
Then, on the Advanced tab:
After completing the above steps, give your Macbook a few minutes to consolidate files, and then check back in iMovie to see if your iTunes library is now available. Please let me know!
Hi again Lyle! Did you have any luck after following the steps I posted previously?
I hope to hear from you!