You would edit the one you're using. (There's a "Use Legacy (Vertical) Postbit Template" setting in the adminCP options, under "Help Style & Language Settings"). Basically, if you have all the poster's info on the left side then you're using postbit_legacy. If it's across the top (like this site) then it's postbit.
BTW, in my previous post I orignially had vb4 syntax for adding the variable to the template, but I just fixed it. Sorry if that caused you any problems.