what software should i use for making add-ons?
Nothing special. Each object consist of a dat file where you define the attributes and a png file where you paint the graphics. The dat file is a simple text file with .dat extension. The graphics can be done with a painting program (GIMP, Photoshop, PaintShopPro or Paint) or with a 3D design program (Blender, Wings, 3dsMax) To load the objects in the game, you need to join both files in a pak file. This can be done using Makeobj...you have more info at the Wiki (http://wiki.simutrans.com/).
In the old forum there was a zip with samples of dat and png files for 64 and 128pak (link (http://archive.forum.simutrans.com/topic/07685.0/index.html)) and a blender resource thread (http://archive.forum.simutrans.com/topic/05054.0/index.html)