It's on 1.3.5 not on 1.4+ but in future they will add it if nothing goes wrong in the mods development

If your in 1.3.5 you have to enable the mod and it will turn on automatically. If your on 1.4+ it's does not have the fire system yet and other features but the current features is superb. Here is a link give by the author in the missing important features discussion https://github.com/Mirsario/TerrariaOverhaul/issues/108