The fact that it's a "Weapon" tab makes me suspect that perk might only affect physical damage and not spells. I also just checked https://gist.github.com/anonymous/7590051, and that entry point only shows up in perks dealing with physical type stuff. You could test it pretty easily though by clicking on your enemy with the console open, and typing something like "setav FireResist 100" before attacking them. (I assume 100 is 100% resistance, it might be 1.0 though, you could test it by wearing a piece of resist armor yourself and typing "player.getavinfo FireResist").
If it doesnt work, you might have to go with a Mod Spell Magnitude entry point instead. You could add several entry points to account for different resistance levels on the enemy. Like:
Mod Spell Magnitude 1.2 with condition
Target GetActorValue FireResist >= 5 AND
Target GetActorValue FireResist < 20
Mod Spell Magnitude 1.4 with condition
Target GetActorValue FireResist >= 20 AND
Target GetActorValue FireResist < 40
etcetera.
Or there might be another way to reduce the opponent's resistance someone else can come up with.