how do you make add-ons? ='(
You learn the TorqueScript programming language, look it up. Notepad is used for writing down the language in a matter of code (though it could be done with other programs).
If you make things such as custom weapons/vehicles/etc., you learn to model with a modeling program, look it up.
There is no program that just puts add-ons together simply, you have to do it all manually.