Placing in notes on top of other notes without removing what was already there.

I am trying to figure out how to add new notes in on top of old notes without replacing the old notes. No matter what I try, the old note disappears when I try to add a new note in. I'm loving the program overall so far, but this one thing is pretty debilitating. Any suggestions? I am using an M-Audio Oxygen 61 MIDI keyboard for input, and I am running Windows 7 on a Dell Inspiron E1505. I have MuseScore version 0.9.5 revision 2012.

Thanks in advance for the help!