What you can do is go to the settings file and manually edit the values to remove that location.
1) Close rekordbox, open Finder and select Go > Go to folder... and enter:
--
~/Library/Application Support/Pioneer/rekordbox/
2) Open the rekordbox3.settings file and find this line:
--
<VALUE name="pioneerDirectory"
3) Change the value from whatever it is now to:
--
/Users/<yourusername>/Library/Pioneer/rekordbox
This should reset it to the default location.