Because you're loading in the swf from another file, the swf doing the loading is now the "_root". So when you refer to root, you're calling the loader shell file, where the score doesn't actually exist.

Instead of using "_root" you can try using "this" or "_parent" until you reach the level where your variable exists.

In AS2, which it seems you're using, it would go something like this...

Code:
on(press){
   this.scorer.score+=1
}
or

Code:
on(press){
   this._parent.scorer.score+=1
}
or

Code:
on(press){
   this._parent._parent.scorer.score+=1
}
until you reach the level where your score is. Not sure how many movieclips deep your button is nested.