I think the problem is here :

var groundType = int;

this should read :

var groundType : int;

And where you say :

while (true)

this makes no sense, there is no boolean called true, so what is the 'true' you are checking for?

but I can see you possibly having problems with groundType not having a value during the start function where you are checking if it == 0 || 1 . And do you want to only run this at start?

Also at that part of the script, both conditionals in your while statement are the same :

if (controller.isGrounded && controller.velocity.magnitude > 0.3) {

did you mean for one to be

if (! ...? (not), 

or detect different controller.velocity.magnitude?

also it would be good practice to typecast audioStepLength also :

var audioStepLength : float = 0.3;

Just in case other people have the same problem that I had, since it can be really hard (atleast for me) to find a working footsteps script, I will post it here. This works fine for me when put on the First Person Controller. The audioStepLength determines the space between each sound, and you can input sounds by increasing the element size in each section. To work you must place colliders near the ground that are set to is trigger, and tagged with the respective tags. I don't take any credit for this since it was mostly fusing a bunch of scripts from the tutorials and from Unity Answers.

var walkSoundsWood : AudioClip[];
var walkSoundsGrass : AudioClip[]; // fill this array with the sounds at the Inspector
var audioStepLength : int = 0.3;
var groundType : int;
var isPlayerWalking : boolean;

function Update(){
    //Check to see if the player is walking by checking for input from the walking keys
    if(Input.GetButton("Horizontal") || Input.GetButton("Vertical")){
       //if those keys are pressed, the player must be walking...
       isPlayerWalking = true;
    }
    else{
       //if those keys aren't pressesd, the player can't be walking.....
       isPlayerWalking = false;
    }
}

function Start(){
       while(true){
         if (isPlayerWalking == true && groundType == 1){
            audio.clip = walkSoundsWood[Random.Range(0, walkSoundsWood.length)];
            audio.Play();
            yield WaitForSeconds(audioStepLength);
    }
          else if ( isPlayerWalking == true && groundType == 2){
            audio.clip = walkSoundsGrass[Random.Range(0, walkSoundsGrass.length)];
            audio.Play();
            yield WaitForSeconds(audioStepLength);
   }
        else{
          yield;
        }
    }
}


function OnTriggerStay(type : Collider){
    if (type.tag == "Wood"){
      groundType = 1;
      print("Wood");
    }
     else if (type.tag == "Grass"){
       groundType = 2;
       print("Grass");
  }
}

I hope that it helps someone! Just get rid of the prints at the bottom and it should work!