It might help if you posted what procedure you are trying to fly and perhaps some screen shots to show how you have the G1000 set up. As the previous reply said you have to lower the AP target altitude to at least that of the next Vnav target and press the vnav button so that vpath is displayed in white at the top of the PFD. Once you get closer to TOD the vpath magenta marker will begin to centre and once centred the white annunciation will change to green.