Variables are not affecting the pages

[moeraccoon]

I'm new to hugo and I'm trying to figure this out, I was reading the page about variables and found something I wanted to enable but I don't understand where I should be adding them.
I see in the FAQ that I'm supposed to edit config.yml, which I can't find, searching here I found someone saying that they used hugo.toml so I used a yml to toml tool but even then I can't get the next and previous buttons to show.
What I added to hugo.tml is as follows:
ShowPostNavLinks = "true"
I tried the same line with ' instead of " and I also tried it without anything. No matter what I change, the entries don't show the option for next/previous.
I would appreciate help a lot.