Hi Luke...!
Just tried it out...no problemo...

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Connect to the Server in FS 2004 first....that's OK...
Start FS Nav...
Go to "Options" in the centre of the screen...
Select "Multiplayer Connect"...
Window pop up..."Continue?"....yes...
This window then appears with the servers address shown...
...add your name....XXXRADAR....then press join....
Tick the box "Public Flightplan"...so others can see your flightplan..!

All other players then appear on the FS Nav map....
....cool....

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Mine works fine every time...!!
Cheers Luke...
Paul...G-BPLF...!
P.S. mustn't forget....my greatful thanks to Sam for showing me how to do it...

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Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.