It depends how you have set the data output. In your simulink file go to: Simulation<Configuration Parameters<Data Import and Export and you can change the options. It also depends whether you are running the simulation from the Simulink interface (pressing Play) or with the sim command. In the latter case, this code can be useful
%Run simulink simulation
simOut=sim('simulink_file',t);
%%Data extraction
outVariable=get(simOut,'nameOfTheVariable');
y=outVariable.signals.values; %output
