When you go on the Character Abilities screen, you will see some have a small 'o' after investing 1 point into
skills with slots. And when you highlight it, there is an option (I think middle, bottom of the screen) that says "Mutate." Press the designated button and it will show a list of your mutagens that you can apply. Once you apply it, you cannot remove it so keep that in mind.
And from the same Wiki, more info about mutagens: http://witcher.wikia.com/wiki/The_Witcher_2_mutagens
