Given, a two six-sided dice are rolled and the two numbers showing are added to produce a sum between 2 and 12.
To simulate a roll of a n-sided die, use the randi() function. You can generate two random integers between 1 and 6 (inclusive) with one function call.
Then all you need to do is use sum() on the results of randi(). This assumes the dice are fair.
You could do this in one line if you'd like
sum(randi()); %you fill in the parameters of randi