I looked at your file:
1) you must give your dynamic textbox an instance name "theScore".
2) Then delete _root.counter from the variable box in the options
3) create a var counter=0; in the actions layer of your main timeline on frame1
4) On the spaceship MC<player>, in the onClipEvent (enterFrame) you will need to put this code:

_root.theScore.text=_root.counter;

this will make your score box work correctly.