Can I make a ship appear in the scene menu?
If you have made a vessel, you will have to make a scenario (.scn) for it if you want to load Orbiter straight into the scenario. Just find a suitable scenario file, copy it, edit the path of the vessel configuration file in scenario ([vessel name]:[config file location in Config folder]), and save your changes. If you now launch your scenario, you should see your new spaceship replace the position of the vessel you edited out.
From other scenarios I can not load them in the menu.
Yeah, a scenario file defines what spaceships will be in the scenario when you start Orbiter. If you go to Modules in the Orbiter Launchpad and activate Scenario Editor, you can press Ctrl+F4, open the scenario editor, and add the your vessel to an already running simulation (with some restrictions, when it comes to really complex addons).
Can you put a landing pad on the terrace of a building?
No, buildings in Orbiter are visual only, and do not abide to the laws of physics, as you neither can land on or crash into them. They are only for the eye candy. If you edit the touchdown points of a vessel, you can make i appear that your vessel is resting on a building, but you have actually just defined that the vessel's resting position is x meters above the Earth's surface, a height which you then have made equivalent to the height of the building. This is why it seems like that the Space Shuttle is standing on the launchpad when you run such a scenario, but if you move the location of the Shuttle in the Scenario Editor, you will see that the Shuttle is actually just "hoovering" over the ground at the same height as the LC39 launchpad.
I have created an Air Base and I want to publish it, but I do not know how to do it
Copy the files you have created into Orbiter file structure folders (e.g. into folders like "Config", "Textures", etc. Just download any addon and see how previous people have done it). Then make a zip (highlight folders in Windows Explorer, right click and press "Send to compressed (zipped) folder". Give the resulting .zip file a descriptive name, log onto / create a user on Orbit Hangar, upload the .zip file wait for it to be approved (can take some days), and hope that somebody tries out and enjoys your hard work!
