Well, it's so simple. You have integers in range 2..12. Declare an array of integers with size 13 (you may use indicies from 0 to 12). Initialize it with zeroes then add 1 to array[sum] element in the simulation main loop.
Use rand() with a proper scaling to get values in range 1..6 (see rand help, use RAND_MAX value).
That's all.
Of course, you are familiare with basic C++ declarations and statements...