You have already implemented all the methods of Die in the header, so there is nothing left to put into the "die.cpp" file (except perhaps the definition of unsigned Die::mInitiator?).
You would just create Die instance(s) in main() (or any other function you like) and use it like Elysia has shown.
By the way, it seems that your effort to reseed the random number generator for each Die object is rather useless. rand() relies on global state, so seeding it in one instance of Die resets the seed for all other instances of Die. You might just call srand in main() at the beginning of the code, remove the initialize function (and the static variable) and simply call rollDie in the constructor of Die instead.
