to compile 

 make

  to build the mesh do 
FreeFem++ square.edp 

 Work try to code the computation of the produit matrix vector explain in the cours. 
function 
 

./mainMesh  Th.msh

  To see the result  of mainMesh with gunplay: 
 gnuplot 

gnuplot> splot "m.gp" u 1:2:3 w l, "m.gp" u 1:2:4 w l

gnuplot>