You need to purchase it, normal price is 3000 emeralds but sometimes there are discount to.
To do this click on thrall menu & you will see 4 thrall (2 activated, as you claim you have them already)
on 3rd & 4rth you will see + button click that and it will take you to purchase section.
Or if you are investing $, wait for some good offer and you can get it their
Did you hire the thrall or did you purchase it?
Hiring is different & purchasing is different.
If you are sure you purchased this and haven't got it.
Submit a ticket here