This is a class method. You need to create an instance of the class, and then apply the method to the instance. See how the tests do it:

plugboard = new Plugboard("AB")
Test.assertEquals(plugboard.process('A'), 'B', "Invalid translation")

First, create an instance plugboard of the class Plugboard, then apply the method to it nameOfInstance.process(... (you could call the instance with another name).