You need to place hold on before you add a second plot to your axes. Your code currently replaces the plot with the stem plot.
Your plot only goes to 2e-3 because that is what you define tt using mod. Perhaps you meant to use t when plotting?
tmax=4e-3;
T=2e-3;
fs=6e3;
xmax=2;
f1=0.5e3;
f2=1.5e3;
t=linspace(0,tmax,2001);
tt=mod(t,T);
x=xmax*chirp(tt,f1,T,f2);
n=fix(tmax*fs);
td=(0:n-1)/fs;
tdd=mod(td,T);
xd=xmax*chirp(tdd,f1,T,f2);
plot(t,x,'b'); % changed tt to t
hold on
stem(td,xd,'r*');
hold off
