This may not be your problem but you appear to be trying to modify a node upstream (Merge2) with media that is downstream. As far as Merge2 is concerned Actors does not exist. I think it is Merge1 Center you should be modifying.
Thanks for your reply. The whole thing is confusing to me still. From the Visual FX Guide, it seems to confirm what I am doing even though nothing makes sense right now. It says:
Tracking the Sky into Position
As you play the composite, it looks fine when the image is still, but as soon as it plays and
the camera moves, the illusion falls apart. For this sky replacement to work, the sky needs
to move as the camera moves. Instead of using the Tracker node as you did in the previous
lesson, you’ll apply the Tracker as a modifier.
1. Select the Merge2 node that is connected to the Sky node in the Node Editor.
You’ll need to apply the Tracker modifier to a node that contains position controls.
The Merge tool includes center X and Y controls that can be used to reposition the
clip connected to the foreground input.
2 In the viewer, right-click over the Merge’s center onscreen control, and from the menu
choose Merge 2: Center > Modify With > Tracker position.
The Tracker modifier is attached to the Merge’s center X and Y controls. Although the
process of tracking is the same as the Tracker node, the Tracker modifier is a singlepoint
tracker that tracks up and down and side to side movement, so it is more limited
than the multipoint tracker you used in the split screen. Still, it is perfect for quick,
simple tracks, as we have here.
Adding a Tracker modifier displays the tracker pattern and search rectangles over the
center position in the viewer. Since it is a modifier, the tracker controls are displayed in
the Merge2’s Modifiers tab.
3 At the top of the Inspector, click the Modifiers tab to display the tracker controls.
The Tracker modifier assumes that you want to track the background of the Merge
node. In our case, we want to track the ACTORS node.
4 From the Node Editor, drag the ACTORS node to the Tracker source field at the top of
the Inspector.
NOTE As you drag a node from the Node Editor into an Inspector source field,
the node will appear to move at first, but it’ll snap back into its original location
once the mouse pointer leaves the Node Editor.
5 Move the playhead to the start of the render range.
Just as you did in the previous lesson, you need to position the Tracker over a high contrast
pattern that moves precisely how you want the sky to move. For this shot, the
ridge in the background has a few rocks that stick out. These would make good high contrast
tracking points that we can use for the Tracker.Then it goes on from there.....
Is the guide just not written clearly or is it me? Does this make sense to anyone else? I'm totally lost on this at the moment and feeling thwarted, as if I'm getting nowhere, but it's only temporary.