Official comment
Sounds like you've got a different username on the new computer - it's an easy fix.
First things first, close rekordbox.
Next, you need know the path to your home directory. You can find it by opening the System Preferences > Users & Groups applet.
If you need to, click the lock icon at the bottom to unlock the panel so you can make changes. Hold the control key while you click on your username from the list and select Advanced Options... your home directory path is going to be /Users/something
DO NOT change anything! Once you know your path, click Cancel.
Open a Finder window and go to the menu Go > Go to Folder... and in that window, copy/paste this:
~/Library/Application Support/Pioneer/rekordbox/
Right-click on the rekordbox3.settings file and select Open with... and select Text Edit.
Before you make any changes, open the Text Edit Preferences and make sure you disable the Smart Quotes, then click the red ball to close the preferences.
Next, use the Find to locate these lines:
<VALUE name="pioneerDirectory"
<VALUE name="masterDbDirectory"
<VALUE name="itunesLibraryFile"
<VALUE name="bridgeImportedLibraryFile"
<VALUE name="SamplerSaveDirectory"
<VALUE name="RecordSaveDirectory"
If they don't exist, that's fine - you should have the top two, at the very least.
In that line, it will have a val and the data for that will be the path to your OLD user's files; you need to update that. For example:
<VALUE name="pioneerDirectory" val="/Users/PulseOLD/Library/Pioneer/rekordbox"/>
would become
<VALUE name="pioneerDirectory" val="/Users/Pulse/Library/Pioneer/rekordbox"/>
Save the file and re-open rekordbox.
Let us know if that works for you.