Click on one of the armored train turrets located on the bottom-left part of the interface, above the minimap. It changes the unit display into the list of units available for construction.
Then when you select a unit, the map shows in green color where you can build it.
Just down left of the main screen, you just click on the gun turret...
Then the colors on the map change (so does the menu) and you can select the units you want and drag and drop on the grenn region you wish...
Don't apologize, you are not the first one you ask (so does me ^^)
English is not my mother tongue too so ask again if its not understandable