The first sentence reads.
"Sectarus is a non-creature familar, which can ce used to cast only curse enchantments."
I would say this means exactly what it says, sectarus follows the familiar rules as explained in the rule book being able to cast but not reveal curse enchantments.
The second sentence reads.
"If this mage attacks and damages a creature with Sectarus, he may immediately cast the spell on that creature as a free action."
To me this is saying that if there is a spell attached to Sectarus the mage may cast it as a free action. The only benefit is the free action, which is a big benefit. I don't see where it says the mage may search his spellbook for a curse enchantment and play it on the creature, which is probably close to how they would have worded it if that was their intention. Since the card says the mage cast the spell I don't think he can even use the mana that Sectarus has channeled.
That is how I interpret the card.