Hi,
I haven't flown the G1000, but typically you only activate the approach if ATC vectors you to intercept the final approach course (it draws an extended magenta line on your mfd).
So to fly the approach from ZONUV you would stay in LNAV or GPS mode then arm vnav which will command the autopilot to follow the vnav profile at TUBTY.
I haven't tried it in the game yet though so there may be bugs.