Official comment
Easy-peasy... with rekordbox closed, open the Finder, select Go > Go to Folder...
Go to this location: ~/Library/Application Support/Pioneer/rekordbox/
Find the following files:
rekordbox3.settings and rename it rekordbox3.oldsettings
browseSetting.xml and rename it browseSetting.oldxml
FxUnitSettings.xml and rename it FxUnitSettings.oldxml
PadFxSettings.xml and rename it PadFxSettings.oldxml
PlaySettings.xml and rename it PlaySettings.oldxml
SamplerSettings1.xml and rename it SamplerSettings1.oldxml
VideoSettings.xml and rename it VideoSettings.oldxml
Open rekordbox and those files will be rebuilt from scratch.