Actually it's made in C++
It loads the Java virtual machine and his own Java applet loader that modifies bits and pieces (I believe the canvas class to give him access to colours, as well as adding stuff to send faked mouse and keyboard events). Of course, it's a bit more complex than that.
One way to look at it though is that Smart in itself is a macro program and then the C++ plugin is just a wrapper to let Scar use it. By itself, Smart can actually macro on any Java applet game, but I don't think anyone has really properly tried so we just assume it's true.
By reading this signature you agree that mixster is superior to you in each and every way except the bad ways but including the really bad ways.