When you use static event handlers like onClipEvent (enterFrame), the scope of the variables is the clip itself. That’s why when you write _currentframe, what it means really is this._currentframe. Fine. But that’s why it also means that Flash is looking for this.framenum, which obviously doesn’t exist.
So you have to specify the right path to get to the textfield, in this case _parent.framenum, or _root.framenum