I've looked at the code, and I still can't figure out how to get into it.

When you enable the mod, there should be a button "show / hide Blueprint Engineer" at the top edge of your screen. This button toggles the GUI where you can execute the actions described in the mod description
Does this answer your question? Or is your problem something else?

So, when I started a new game with it enabled, I saw it. But when I added it to an existing game, it didn't appear. :-/ That's kind of strange.

Looks like I didn't remember defining the on_init-function properly. I'll make an update as soon as I find the time to do it