There are two different ways of running spice simulations.
1) Generate a .cir file from the meno @Rino showed and start ngspice with the .cir files as an argument.
2) In the nightly builds there is a new feature to start ngspice (as a library!) directly from EEschema, as others have replied above.
The two ways of working are different, but both work. It is whatever you prefer.