I got the thing to work now using pstudio's suggestion.

In case someone else is looking for something similar:

[pascal]Type TMyMouseClass = Class
procedure MouseUp(X, Y: Integer; Button: Integer);
end;

procedure TMyMouseClass.MouseUp(X, Y: Integer; Button: Integer);
begin
// your mouse stuff goes here
end;

(..)
// add this to the oncreate, onInit or whatever, of your game.
with TMyMouseClass.Create do
begin
TPHXInputListener.getInstance().addMouseUpHandler( MouseUp);
end;[/pascal]