There is no loop, because k only gets value of t-t/2=t/2 and none of the preceding ones. Changing that to k=1:t would give you the looping the way it is expressed here on the Wiki page.
The formula then needs to change to take either the first or the second term, depending on whether the current k is even or odd.
function [z] = wallisproduct(t)
z = 1; % initialise z to be used for the multiplicative steps
for k = 1:t
b = floor((k+1)/2); % find 'half' of k: 1 for 1 and 2, 2 for 3 and 4, 3 for 5 and 6, etc.
if mod(k, 2) % if k is odd, then mod(k, 2) is 1 which is true
z = z * 2*b /(2*b-1); %
else % if k is even, then mod(k, 2) is 0 which is false
z = z * 2*b /(2*b+1);
end
end
end
This then gives:
wallisproduct(4)
ans =
1.4222
