There are a number of ways, one of which would be...

    String ss[] = "123".split("");
    int i = 
            Integer.parseInt(ss[0]) + 
            Integer.parseInt(ss[1]) + 
            Integer.parseInt(ss[2]);
    System.out.println(i);

another would be...

    String s = "123";
    int i = 
            Character.getNumericValue(s.charAt(0)) +
            Character.getNumericValue(s.charAt(1)) +
            Character.getNumericValue(s.charAt(2));
    System.out.println(i);

and still another would be...

    String s = "123";
    int i = 
            s.charAt(0) +
            s.charAt(1) +
            s.charAt(2) - 
            (3 * 48);
    System.out.println(i);

BUT hard coding for 3 numbers isn't very useful beyond this simple case. So how about recursion??

public static int addDigis(String s) {
        if(s.length() == 1)
            return s.charAt(0) - 48;
        return s.charAt(0) - 48 + addDigis(s.substring(1, s.length()));
}

Output for each example: 6

The immediate problem is due to you using <= temp.length() instead of < temp.length(). However, you can achieve this a lot more simply. Even if you use the string approach, you can use:

String temp = Integer.toString(guess);
int[] newGuess = new int[temp.length()];
for (int i = 0; i < temp.length(); i++)
{
    newGuess[i] = temp.charAt(i) - '0';
}

You need to make the same change to use < newGuess.length() when printing out the content too - otherwise for an array of length 4 (which has valid indexes 0, 1, 2, 3) you'll try to use newGuess[4]. The vast majority of for loops I write use < in the condition, rather than <=.

public class Kata {
  public static int[] digitize(long n) {
    String numStr = String.valueOf(n);
    int digits[] = new int[numStr.length()];
    int counter = 0;

    for(int i = numStr.length() - 1; i >= 0; i--) {
      digits[i] = Integer.parseInt(Character.toString(numStr.charAt(counter++)));
    }

    return digits;
  }
}

int[] test={10212,10202,11000,11000,11010};
ArrayList<Integer> test2 = new ArrayList<Integer>();


for(int i = test.length -1; i >= 0; i--){
    int temp = test[i];
    while(temp>0){
        test2.add(0, temp%10);  //place low order digit in array
        temp = temp /10;        //remove low order digit from temp;
    }
}

This will do exactly what you want by placing the lowest order digit of an entry into the "front" of an arraylist, and therefore in front of the previous low order digits/entries.

If you need it to be in an Array, ArrayList has a toArray method.