Most of the mods are just unzipped into the existing Orbiter folders.
There is a default set of folders where each item has to be placed. i.e. Meshs go in a folder called MESHS. The zip file will have these folders and the required files sitting in them.
So if your orbiter install is in a directory call C:\orbiter there will be a c:\orbiter\meshes, a c:\orbiter\textures and so on. Just unzip the file to the orbiter directory and all the nice new files will drop into place. Next time orbiter is run, it will pick up the files as if they where part of the install.
Some other add-ons are more complicated but 99 out a 100 come will full instructions.
Lastly some add-ons are in fact addons to addons and require another file to be installed first. Again 99 /100 tell you just which one to be downloaded first.