How to use the mod sequentially upgrading extractors

It sounds great, I found plenty how posts telling how great it is. Problem, no one ever explains how it works and I can't figure it out. I never manage to upgrade anything with this mod.
Could a kind soul unveil its secrets to the world, please?

Ok, looks like I was running an outdated version of the mod that simply didn't work any more…
But just in case: to make it work, select all the upgradable units you want help with in the order you want them upgraded and click the green button the mod adds on the top of the screen.