There are two different types of postbits: vertical postbit and horizontal postbit (also called legacy postbit, as it was the first generation postbit); each type of postbit has its own template: (respectively) postbit and postbit_legacy.
Your forum is using the legacy postbit, so any modifications you want to make to your postbit need to be done in the postbit_legacy template.
What I'm trying to get at here is that the reason you're not seeing anything is because you're editing the wrong postbit template.