Changes to Mod.java and GuiModList.java (more info bellow)

+GuiScreenClass() in Mod.java
(The classpath to the GuiScreen class)
+GuiScreenName() in Mod.java
(The name that will be displayed on the button)
+2 functions in GuiModList.java to make it work
*func_73866_w_() in GuiModList.java where I added another button (you
might want to change the ID)

Feel free to modify some code as this is my first contribution to
ForgeModLoader