I am VERY new to Fusion (although experienced with After Effects, and some experience of Flame and Avid DS as online tools - not a total beginner with vfx/finishing software!), and have what I suspect is a bit of a beginner question. However, it's one that I haven't been able to find an answer for by Googling.
I have a very simple set up, made from following the first 'Introduction to 3D Space' video on the Blackmagic website. Why can't I connect the output (little red square is what I'm trying to connect from) of the Duplicate3D node to the input (little green triangle is what I'm trying to go to) of the Merge node?
Here's an image, which hopefully will help clarify my poor explanation!

Any suggestions much appreciated.
Using V8 build 23 of Fusion, on OSX 10.11.3.
Any tips of good learning resources would be great too, I clearly need them!
Cheers,
Michael