In order to run an MPI compiled program, you must type:
mpirun -np <number of processors> [mpirun_options] <program name and arguments>
where you specify the number of processors on which you want to run your parallel program, the mpirun options, and your program name and its expected arguments.
Some examples of mpirun's:
mpirun -np 4 hello
mpirun -np 6 -machinefile machines hello
mpirun -np 5 integrate 0 1
Type man mpirun or mpirun -help for a complete list of mpirun options.