I have over 60 hours of gameplay and I still didn't find out how to use those mutagens ><

Can someone please explain how to use them? (what should i buy to unlock them?)...

You should see abilities with small lil' circles attached to them. Right click on that ability.

There are a couple of skills in the skill tree that have slots for them, like 3 last skills in swordfighting, 2 last in everything else. You can select a mutagen for that slot. Easiest way to see this is 2 topmost skills in the tutorial tree, you probably have unlocked at least one of them.

Thanks I managed to mutate it ^^

Can you swap out the mutagens in the slots or are they permanetly filled?
Are all slots(armor/weapon) refillable I guess is what I'm asking?

Permanent. It's recommended to sell the lesser and normal mutagens and only use the Greater mutagens.

They're permanent. Not all abilities can be mutated, only specific ones.

Thanks scrooge here, Is there a list of mutagens/enhancment yet? (Besides Viagra and Smiley Bob Extenz stuff)