As @Killer Angel says, "attack characteristics" means "number of attacks". So he gets +6 attacks with his weapon, not +6 to hit rolls. To be honest, the term "attack characteristics" is terribly confusing though.. you're not the first nor the last one to be confused by that term. Why they can't just say "number of attacks" is beyond me...
Firelance increasing in price was a bit of a head scratcher. Requiring three units of knights to fill it out comes with the implication that they will basically be the 'bread and butter' of your army but they lack the staying power or the offensive output for a +3 to charge to be meaningful. Not to mention the godawful blazing lance's which more often than not don't even activate before the knights die. Have the firelance grant +1 to hit and wound to knights while close to the Scar Veteran and then I'll consider dropping 150 on it.
I play with at least 10 knights in every game due to the limited models that I have and I don't recall ever causing even one mortal wound from the 6 on wounds.
It should trigger once in every 18 attacks after a succesfull charge, without modifiers. Seeing as we only get 1 lance attack per model, and we won't be charging every turn it's entirel possible to go multiple games without it ever triggering, so I'm not surprised… To be honest, I kinda wish they'd just remove the charge requirement. It'd still be a rather unreliable and weak ability, but at least it'd probably trigger at some point during a single game instead of nearly never...