If you want a fader for a soft-synth on a track then you do need to add one (or more) in the modify busses dialog. You can then assign that output to the track and the fader will control the amount of MIDI volume (default - controller 7) sent to the plug-in. Now this is not the recommended way to use a soft-synth unless the soft-synth only accepts data on a single MIDI channel. If the soft-synth accepts data on more than one channel or receives audio it is recommended that you install the soft-synth on an aux buss.
Assuming that the soft-synth is installed on a track, the output from the soft-synth will initially go to whatever audio output the track was assigned to before the plug-in was installed. Afterwards you can change the output assignment in the effects window. There was a bug where this would not work until stop was pressed and then playback was pressed again. This is fixed in 6.1.0.6.
Here is a simplified flow chart for Metro:
In the above picture, the output (instrument/mixer) fader after the effects chain is specified in the effects window.