To use the rekordbox Cloud Option 1TB powered by Dropbox feature, you'll need to:

• Cloud Option.
• Create a Dropbox personal account.
• Join the Cloud Option DJ team (from My Page on rekordbox.com)
• Upgrade rekordbox to the following versions.

rekordbox for Mac/Windows ver. 6.8.0 or later
rekordbox for iOS ver. 4.1.0 or later
rekordbox for Android ver. 4.1.0 or later

Find out more details here.