Update:
I finally found out how it works.
Units that require a certain ressource to train only appear in your training selection list if you have AT LEAST ONE of those ressoures in your ressource pool.
In my case the Mages did not appear in my training selection list until I had a single crystal.
Once I had a few crystals they were listed along with the Chosen. And if you click on them you see that they cannot be trained because training requirement is 20 crystals for Mages and 36 crystals for Chosen.
Same goes with certain buildings btw.