Hi Pete
this is de design of rekordbox.
When you run rekordbox on a PC/Mac you can link to the CDJ/XDJ supporting DJ Pro Link. The CDJ/XDJ Need to be connected via LAN cable with a Hub/Switch or Router. The PC/Mac need to connect to the same Network either through LAN or WLAN.
Once rekordbox detect the Player it will give you a 'Link' button at the bottom.
After linking you can use the rekordbox library and push tracks to the CDJ/XDJ. If you plug in a rekordbox USB decive into the PC/Mac you can view that library as well.
You don't need rekordbox DJ for this Basic function.
Please refer to the rekordbox Manual for the Details.
Hope this helps.