The quote, "I have love in me the likes of which you can scarcely imagine and rage the likes of which you would not believe," is not found in Mary Shelley's novel Frankenstein. It actually comes from Kenneth Branagh's 1994 adaptation of Shelley's novel.
While the quote is not found in the novel itself, there is one quote which is close to Branagh's adaption. This quote is found in chapter seventeen of Shelley's novel: "If I cannot inspire love, I will cause fear; and chiefly towards you my arch-enemy, because my creator, do I swear inextinguishable hatred."
While the quote from the novel does not directly speak to the love the Creature possesses, as does the Branagh quote, it does speak to the rage the Creature possesses. Given the fact that the Creature is asking Victor to make him a mate, the Creature is trying to impress upon Victor the two choices he (the Creature) has in life: inspire love or inspire fear.