In the book In the Time of the Butterflies, Minerva states: "Adversity was like a key in the lock for me". What does she mean by this?

In her statement that

"Adversity was like a key in the lock for me",

Minerva is referring to how she is most productive when things are tough.  Adversity serves to unlock an inner strength within her, making her fearless and intrepid, completely effective in her work for the Revolution.

After her release from prison, Minerva had gone through a period of malaise.  She had been depressed and edgy, wanting to do nothing but lay in bed away from the stresses of the world.  Memories tormented her, and she was nervous, nothing like her former self.  Then, there is word of an uprising, which fails, "plung(ing) the whole country into despair again".  Paradoxically, this downturn of fortune brings brings Minerva out of her torpid state, and she is galvanized into action, focused once again on the cause most dear to her heart.  Adversity indeed has worked like a key to the lock upon Minerva's spirit, and, with renewed energy and self-possession, she reassumes a leadership role for the Revolution (Chapter 12).