There is an issue with Civil 3D 2015 that has cropped up for several users even those with the latest service pack. The issue occurs when you have multiple drawings open in Civil 3D. You may find that when you try to enter something at the command line, nothing appears. Further investigation might reveal that what you are trying to enter at the command line is actually getting entered at the command line of another open drawing. This behavior has been linked to the ON status of LEVELOFDETAIL.
You may not even be aware this feature is turned on. If you see the icon here in the upper left corner of your viewport, then LEVELOFDETAIL is enabled.
You should disable it by running the command LEVELOFDETAILOFF. If that does not help solve the command line problem mentioned above, then there is another option found here.
2015-06-09 Update: Civil 3D 2015 SP3 claims to have fixed this. Not yet tested here.