Nothing has changed with GSX Level 2 in the way the Simobjects are installed. They are always in the same place, under the Addon Manager\Simobjects\Misc.
The Addon Manager folder, of course, is installed in the location YOU chose when you installed it and, by default, it's under C:\Program Files (x86)\Addon Manager, but can be changed. Since more than one year, we don't install anything in the main simulator folder, if this is what you meant with your question.
if I choose "edit position" nothing is in the airline window
Nothing is supposed to be there, unless you are editing a parking on a airport that already have some airline codes in the AFCAD. On a default airport, it's always empty.
I am trying to add US Airways.
UA
Codes used by GSX can be checked in the SIM.CFG of any vehicle that has multiple liveries, like the FSDT_Staircase, for example.