The function rand generates random numbers uniformely distributed between 0 and 1. Hence, it is enough to set to 1 numbers the value of which is higher than 0.30 = 1 - 0.70, and to 0 all the others.
n = 10000; % = number of values to generate
prob_1 = 0.70;
num = rand(n,1) > (1-prob_1);
num(num ~= 1) = 0;
% Validation
numel(find(num == 1)) / n
numel(find(num == 0)) / n
