You
cannot mimic using those instructions with ISBoxer 42. That video was showcasing new features that you cannot replicate with ISBoxer 1.
From the thread that links the video...
This video demonstrates how to multibox browser games with ISBoxer 2 Alpha, using multiple Firefox windows. We make use of the Window Capture Module and Video FX Layout Modules to briefly play at diep.io
... which, by the way, shows several features new to ISBoxer 2 that you can't do with ISBoxer 1. 
It's not that you need instructions to do it with ISBoxer 1, it's because ISBoxer 2 is using all-new video FX and broadcasting systems natively built into LavishGUI 2, and without those you can't multibox browser games. ISBoxer 1 uses the original LavishGUI, and had to create its own Video FX and broadcasting systems. The new version is, in fact, "new and improved".
You can add "Other Window Sources" to ISBoxer 1, to view your browser window in DxNothing. You can even turn on KB/Mouse pass-thru, and it might work a little depending on the browser and the page. But even if you do that, ISBoxer 1 has no way to control multiple of them at once.
All of that has been revamped for the new version.
The reason you cant find the Window Capture Module without ISBoxer 2 is that it's an ISBoxer 2 Module. I created it and published it to ISBoxer 2 last week as part of a different video. With ISBoxer 1 you are limited to the "Other Window Source" setup as mentioned by altbob