Agree with all those above who questioned the quality of the job - horrible and amateur. If you can get back or withhold payment for labor do so.
Blackjack 57 will not fix this.
@PirateGirl has the correct idea; get some self leveling cement, figure out the highest spot and calculate how much you will need to cover the entire floor area to that depth. From what I can see 1" of depth (perhaps less) should be enough. Mix and pour as evenly over the surface as you can, it should flow pretty much on it's own. Check the setting time on the label to ensure you have enough time to mix and pour the total amount you will need. Two people will make the job much easier.
Self leveling cement should be cheaper than a pre-mixed floor leveler and should be easier to put down.
Best of luck.