i use SSH via WLan to conect to my N900. Dont know much about the USB Networking (Dont need it, never trust USB for something difficult as Networking), someone else will have to help there. I Think USB Networking has to be enabled first to use WinSCP / SSH. This may be a bit tricky.
WinSCP and Putty (putty as remote console)
Read the project pages / howtos for WinSCP and Putty, its nothing specific with the N900.

I Think Mac can use SSH FileTransfer natively. See SSH Manfiles / HowTos.

SSH is really simple to understand (if you stick with Username/Passwd authentification) and has some really noob-friendly documentations.