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You need to "slot" them into talents, certain talents have little circles above em mostly ones near the end of the tree. Go to your character screen and pick one of those talents and you can use a mutagen on it.
I was going to say the cake is a lie, then I took an arrow to the knee...Then I ate some oranges and everything was k.
It's recommended that you don't use lesser mutagens. Apparently on the PC there used to be builds dedicated to using Greater mutigens (via the Impregnation talent from the Alchemy tree) but they've since patched the game to make the Greater mutigens extremely rare. I think you're guaranteed to find 2 Greater vitality mutigens on the 2 Endrega queens (I did on 2 playthoughs), so I used those as well as the Madness mutigen that I found, which is also pretty rare. Other than that I didn't bother with them.