Can a character deal the damage from the Knockdown Rage Power as nonlethal damage?
If so, could they also apply the intimidate check from the Enforcer feat to that attack?
No. The Knockdown power does not specifically state a type of damage so it is lethal - non-lethal damage is always specified. Also it does not specifically state the user can decide to make it non-lethal.
Combat says: "You can use a melee weapon that deals lethal damage to deal nonlethal damage instead, but you take a –4 penalty on your attack roll." But this is not applicable to the Knockdown power because that does not state that your damage is caused from a melee weapon.
Since there is nothing else applicable here, the damage must be lethal.
This all assumes normal combat using just this Rage power - I'm assuming you don't have any other abilities, feats, traits, or items that let you do this with non-lethal damage because if you did, you would have listed them and used them accordingly, probably without needing to ask your question at all.