Yes I did the same myself for 2 tickets in December and Febuary using flight credits.
Log in first and then choose your desired destination and click pay.
You will be asked how to pay and on the right if I remember correctly is an option of using flight credits.
It was very simple and easy to use.
All I can think of is that maybe you guys are not a member of their frequent flyer club.
If not just join and then you can access your flight credits and redeem.
Good luck.
Just another thought is that when my flights were cancelled I had to chat to ava the robot to claim my cancelled flights as a credit to my account, it was not automatic to credit said account.