Truckdown isn't a program, just a POI set as the usual Garmin *.gpi file. Unlock it with [Only registered and activated users can see links. ] if locked. Put it in Garmin>POI folder. If it can be correctly read by SG fw you'll get an additional icon in 'Where To?' menu as shown here: [Only registered and activated users can see links. ].

To set simulator in 765: Tools>Settings>System>GPS Mode and select 'GPS Off'.