Not sure what the "little bit of wire" could have been.
There are two different types of barrel adjuster - one that threads into the shifter/brake lever/bike frame like this:
View attachment 518897
Another which just slots into the shifter, and turning the barrel moves the inner cylinder relative to the outer, like this:
View attachment 518898
If your barrel adjuster looks like the bottom image (which I suspect it will from what you've written), then it just slots into the shifter, and the cable tension holds it in place.
i.e. it doesn't screw directly into the shifter - sorry my earlier posts were probably a bit confusing there, I had the top image style design in my head when writing them.
Confusing stuff, but on the plus side you are learning a lot