To place stationary machine guns, go to the editor menu under West, East, or Empty (if you do not want a gunner or you want to add your own) and go to the Armored category. It should be called "M2 Machine Gun" or "M2 Machine Gun (East)".
If you have version 1.75 (Resistance):
For houses as well as other objects. I would recommend Kegetys' Editor Addon, which you can get from his site, here:
http://koti.mbnet.fi/stede/flashpoint/The other way to do that is by createVehicle'ing the buildings in based on their name, but that is a little more complicated.
By the way, I highly recommend you get some of the other addons from Kegetys' site, like his Hawk, Russian Weapons, and Revolvers.
If you do not have Resistance, look for Gunslinger's editor upgrade, it does practically the thing as Kegetys'.