As noted in the ** rekordbox Read-Me,** the default location for the rekordbox files on Mac is: /Users/yourusername/Library/Pioneer/rekordbox
If you have moved from another version of 10.x to 10.10, you're likely having problems with missing files thanks to iTunes and how things were changed there. If your files are okay within iTunes, you should have access to them over the bridge. Verify the correct location of your iTunes library within the rekordbox settings.