@mark > Is the question how you load a song from rekordbox to a CDJ?
Unfortunately the CDJ-350 is not LINK compatible and can only use rekordbox-prepared USB devices. There is no ability for it to connect to the computer for rekordbox LINK.
But, the CDJ-900nexus does LINK to the software! You can test this by disabling your wifi and directly connecting one CDJ-900nxs directly to the computer's ethernet port. When you open rekordbox you'll see a LINK icon at the bottom left, click on that and you'll see your CDJ. If you want to connect multiple CDJs, you can use an ethernet switch, or router (note that some routers filter or manage network traffic and thus may not show the CDJs within rekordbox). If you don't see the LINK icon, try disabling your firewall.
If you have any other questions or need clarification, please ask.