The game has UGC built into the game (linked with the Steam Workshop) so you can have extra content when you have used all of the games default content. Like stated in one of the developer's dev videos, you can use any 3D modelling program you like. All you need to do when you have finished your model ready for RCTW is download Unity3D if you do not have it and download their plugin. Atari/Nvizzio's plugin allows you to convert that Unity file into a RCTW model file and you can now use it as a decoration model in your park.
Modelling is for experienced users who know how to use Unity and their Modelling software of choice.