I’ve made a robot using the basic biped skeleton and changed the structure a bit (5 fingers and 1 spine). I then used the select and link tool to assign the arms to the skeleton’s arms, legs, etc. I now want to use footsteps but the option remains gray and unusable. It works with other bipeds but not this one. It also worked just fine when I had only assigned the arms to the skeleton. Maybe I accidentally pressed some key combination (There are green squares highlighting the ends of joints that just randomly appeared)? Any help would be appreciated 
And hence further supports my theory that bipeds should not be used. They cause to much trouble than they are worth. I have no idea, I don’t use the biped. I make my own rigs and then loop a walk cycle.
But if i had to guess, make sure you are selecting the root of the biped (that little diamond shaped thingy) And, make sure you are selecting the footsteps that you want him to walk on.