how to adjust rapture modulator node volume ?

Hi,
 
Are you trying to adjust the Rapture Envelope Generator Segment Tracking? 
 
With focus on the Envelope section you wish to edit you can press the "V" hotkey to enable velocity segment tracking edit mode.  You'll see red transparent block come up when you drag up or down between two Envelope nodes.  This translates to increasing or decreasing the time of the segment based on incoming Velocity.
 
With focus on the Envelope section if you press the "K" hotkey this will enable keytrack segment edit mode.  You'll see blue transparent blocks appear when you drag up or down between two envelopes.  This will increase or decrease the time of that envelope segment based on the MIDI note number keyboard range.
 
Of course please make sure that your host allows passing hotkeys to the Rapture VST plugin or else you won't be able to get to these modes.
 
Hope that helps,
Chad