Use the integrator block. If you attached a sine wave block to an integrator block (in the Continuous library), you will get what you want. In MATLAB code, you will get
t = 0:0.01:(2*pi);
x = sin(t);
dt = t(2)-t(1);
y = cumsum(x.*dt);
plot(t,x); hold on;
plot(t,y,'r')
Or you can use the x above and put that in a "From Workspace block"