This block should be used only to reduce the speed of a Simulink® model so that it can be observed running in real time. If the simulation is already slower than real time, this block has no effect.
To use it, extract the files to a directory on the MATLAB path. Re-open the Simulink browser and you should see a new library named "Useful Stuff" which contains one block named "Real-Time Subsystem". Place this block in a model to use it.
To use this file under other operating systems, just run "mex sfun_time.c" to generate the MEX-file for your platform
For accurate hard real-time, I recommend looking at xPC Target or Real-Time Windows Target.