If you want to create your own addons, you should download the Orbiter SDK (software development kit) package. This contains the libraries for compiling your own spacecraft code, sample code, a few utilities and documentation.
As a minimum, you will need to create a mesh for your spacecraft model (look at the Related Sites page for links to mesh converters from popular 3-D model formats such as 3ds), and define its basic physical properties in a configuration (.cfg) file. For more sophisticated models you may need to write and compile a specialised vessel plugin dll. Alternatively, there are 3rd party wrapper dlls available which can be controlled via configuration scripts.
Finally, you must design a scenario (.scn) which loads your new spacecraft to see it in action in Orbiter. It is probably easiest to modify an existing scenario file by replacing one of the vessel definitions.
If you get stuck, you may want to ask for help (or propose a collaborative project) on the addon forum.