sorry guys, you are absolutely right. The "e" here is the all-one vector. The example was just meant to point out the type of argument: I am looking for. Here, as the function is linear (as in being a homomorphism, i.e., f(0) = 0), I can compute the average of the evaluation function by computing the evaluation on the average. Than I can count by multiplying $2^n$.

Thanks a lot guys. That was what i was looking for! --Alberto