The trick to making QLab respond to incoming timecode is that you have to enable timecode at the cue list level first:
Now, cues within that cue list can respond to timecode triggers.
Important: QLab can be triggered by timecode, but does not chase or lock to timecode. Pausing incoming timecode will not pause QLab, and rewinding or fast-forwarding timecode will not scrub cues. QLab just listens to the timecode as it comes in, and if a frame comes along that a cue has set as a timecode trigger, then QLab triggers that cue.