If your cement slab has a crack in it then it would be best to use a concrete sealer first on the cracked cement slab to seal the crack.
Then you can use the leveling compound over the filled in crack on the cement slab to level the concrete slab.
You could pour the leveling compound over the cracked cement slab without filling in the crack first but you'd get much better results filling in the crack in the cement slab first and then applying the concrete leveling compound.