I thought I'd put up this little diagram after having so much trouble finding any directions online of how to properly thread my top-tension treadle machine. As there don't appear to be any existing manuals for my machine, I eventually had to piece it together using old diagrams and bits of directions from manuals for different models of machines. That said, it appears that different top tension models were each a bit unique in their set up, but this should help get you started, at least!
This next bit was a little tricky to figure out. The easiest way to do it is to bring the thread straight down behing the little guard (5), then reach in from the front and pull a little loop of thread towards you through the guard and loop it over the hook (6). All that's left now is to thread the needle (7) from left to right. The few top-tension machines I've seen online so far have all threaded left to right.
That's it! I hope someone else finds this useful!