First you place it (while it's a green box it can be moved), then turn it with the rotation key (it should be R) torwards the direction of the enemy or wherever you want it to be turned at. Then in the commands at the bottom right look for a button that you've never seen before, it has a shovel if i remember well. Once you press that, the green box turns into a bunch of sandbags, thats the "plan" or "draft" for you mgun post. Remember that once you've set it you can't turn it anymore, so before you do that be sure that the mgun is facing the right way. From the green box you can't really tell where it's facing but the rotation command faces it in the right way by simply clicking in the direction you prefer. Now you can assign a max of 4 soldiers to the draft, with the men selected just click on the project and they will start building. Wait for them to finish and you're done :)