Leveling concrete

I am in the process of remodeling my kitchen and I want to put engineered wood flooring down. My problem is that I had a 4' addition put on the kitchen and the new floor isn't exactly level with the old floor. The house is built on a concrete slab so the floor is concrete. I have heard of products that are suppose to be self leveling but I don't know anything about them let alone using them. Can you please advise me on the best plan of attack for this project?