The original poster asked for the discrete time signal not the continuous time signal. A discrete time signal just means sampling your continuous signal at discrete time intervals.
The simplest way this can be done is by increasing your step in n.
n = [-5:0.25:5];
y = 5*cos(pi*(n/2)-(pi/2));
stem(n,y);
The proper way to do this would be determining a sampling rate and implementing it in your code.
