Splinter Cell: Conviction. Mark & Execute....xbox 360?

You have to execute someone by hand first. It also has to be stealth kill. You can't just walk up in front of him and punch him in the face. Stealth kill and then you have mark and execute options. So, press RB when your reticle or crosshair is on an enemy. Once you've marked whoever you want then press Y. Remember, that there's a limit to how many people you mark and it also will alert others when they fall. So think carefully about who, when and where you want to execute. It's actually really simple. It might be because your gun of choice doesn't have the mark and execute feature? Not sure though because I haven't encountered this problem. It might be your gun. Try choosing another gun and try it. If it doesn't work then I'm not really sure. Just make sure you've got a stealth kill, and marked people with RB, then press Y to execute. Hope this helped!