Yeah, I think we need to fix up the manuals a little more on this
You can add one to a game in the same way you would add a new league to a game. If you go to the game settings page, there's a button for actions in the upper right which should have a few choices "add a new league" "add a new tournament".
Then you step through the options for a tournament. Need to decide which type of tournament you want - WBC-style, Champions League style, or Arizona Fall League style would be essentially the 3 main "types" of tournaments, although there's a ton of customization on top of that. The manual I believe should go into some detail on many of the option choices. If you want to start out, I'd suggest adding a WBC-style tournament, seeing how that plays out, and then you can explore within it to see different options available.