Make sure you have got Hotsync running on your desktop machine with the option to sync over network ticked (right click the hotsync tray icon and make sure there is a tick next to network)

The IP address you need to enter is the local IP address of your desktop machine - probably 192.168.1.something. You can find out what it is if you don't know by clicking Start - Run and enter cmd
then type ipconfig and checking the IP address shown for Ethernet Adapter local area

Don't try putting in your host name - it is unreliable.

As long as you are connected to the same LAN as your N800 this should work. Works for me....