Processing will only look in the main PApplet class (pde tab) for the method mousePressed() so it wil not find it in another class.

registerDraw() in another class registers objects from that class with the draw loop. In other words the class must have a void draw() method. Note that is public void draw() if the class is in a java tab.

To register the class for mouse events  you need to use registerMouseEvent() and have a method void mouseEvent(MouseEvent event) in the class.

I should also mention that the register??? methods have been deprecated so are likely to dissappear (no timescale annouced yet) in favour of the new registerMethod()

HTH