Hello and welcome to justanswer, my name is ***** ***** i will be assisting you today,
Please Open iTunes, from the menu bar at the top of your computer screen click File > Library > Organize Library and select "Consolidate files." Then click OK.
Once that process completes, open iMovie and see if you can add the song.
Ok, reopen iTunes and then click the Account tab at the top then select authorize this computer.
After the authorization has completed, check iMovie again.
let me know the results
I'm sorry that the solutions I provided did not restore access to your iTunes library, please try this :
Open the Terminal app in Applications then Utilities folder and copy and paste the command below into the window that opens then hit Return:
Then reboot and try iMovie
If the problem persists Backup you your data, Reboot into the Recovery HD by choosing the Apple menu > Restart (or hold the power button for 10 minutes until it shuts down.), then hold down the Command + R keys on the keyboard during the start up process. start it up again holding those keys. When you see the Apple Logo you can let go.
When the computer boots, choose your language then you will see the OS X Utilities screen. Select the Disk Utility and then Continue. Then in Disk Utility select Macintosh HD in the left column (if there are two with that name or it is named something else choose the first indented item) and then on the right select the First Aid tab then select Run. If that is ok quit Disk Utility and select Reinstall Mac OS X.
after restart open Terminal and copy these lines and hit Return after each then open iTunes first then open iMovie:
Has everything worked out, do you still need help please?
If you need further assistance please reply back to me and i will gladly continue to help