OnMouseDown Event

In 9.3 to get the XY of a mouse click i'd use:

Public Overrides Sub OnMouseDown(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Integer, ByVal Y As Integer)

But in 10.0 I get the following error:
sub 'OnMouseDown' cannont be declared 'Overrides' because it does not override a sub in a base clause.

What does this mean?? and Whats the best/easiest way to get the XY of a mouse click??