On the main page you can type in the registration number and if it is not in the database it will give you the option to add the aircraft.
Then you have two pages to enter as much information as possible about the aircraft. Don't be concerned if you do not have all of the information. Just try and be correct with what you do have. If it says you must enter a numbe rjust enter 0000.'
Have fun!
and Welcome to AP-Data!