If you need a numerical result, the sigma notation turns into a call to the sum() function. Your example might look something like this (with random numbers substituted for your data):
N = 100;
fn = rand(N, 1);
Fn = sum(fn);
Note that MATLAB array indices start from 1, so although in your formula the index j runs from 0 to N-1, in MATLAB the vector index runs from 1 to N. You may need to take account of this when you compute whatever is to be stored in fn.
(I assume that "Fn=" is on the wrong line of your formula, which should look like
If you want a symbolic result using the Symbolic Math Toolbox, then someone else will need to help.