A frequency array is simply going to be an array where the index is the value and the value stored at the index is the number of times that value has occured. For instance,
So, in your program when you roll a number you do something like this:
int frequencies = new int;
frequencies; // represents the number of times a 1 was rolled
frequencies; // represents the number of times a 2 was rolled
frequencies; // represents the number of times a 3 was rolled
Hope this helps.
int rollValue = Die.roll(); // or however you are getting the value fo the roll
frequencies[rollValue-1]++; // increment the number of times that the value showed up.