Tolk: you are using composition -> public var
So there is no need to extend the MovieClip. Only extend the
movieclip class when you need all or most of the movieclip
functionality. Your code has a scope problem:
mc.onRollOver=Over(); // btw, should be mc.onRollOver=Over;
The function Over() is now in the scope of mc and you want it
to be in the scope of the current object (the instance of ToolTip).
Use the attached code.