For determining which numbers in the range are even, a modulus would work. As for std::accumulate,Most C++ courses now are using namespace std; I believe.
What is your code so far? if you don't post what you've tried, people tend to think you want them to do your homework for you.
I'll give you some hints as to how I would write this:
in main, declare an int sum variable initalized to 0, an int number variable initialized to 1, and an int limit variable initialized to 100.
use a while loop to continually iterate to limit, with a modulus operator inside to check if number is even.