xp10?
1. go to joystick & assignment
2. go to buttons adv or tabs key. Depending you want to map a key or a button
3. you press the button you want to map (or you select the key you want to assign)
4. on the right side, you clic on the top right corner "custom cmd field" and a tree will popup. Navigate to ixeg stuff and select whatever you are looking for there. i.e. 'a/p disc'
xp11 works the same but the UI is different and there is still parsing issues for the information to be shown properly. You need to map / un map blind at first, and then items will populate on the list.
note:
It is quite common people confuses Custom Commands vs Custom Datarefs or think they are the same. They are not, both lists are documented under the "2-interface guide.pdf.
Custom commands. These are actions the developers provide for the users to map to key/buttons directly. So when you invoke them via button or key press, they perform an action pre-programmed.
you use custom commands via mapping as explained above.
i.e. 'a/p disc' when you map this custom command to a button or key, when you clic it will disconnect the autopilot
Datarefs. These are memory positions the simulation store values. Based on the value the simulation does/show something.
You can not map datarefs under joystick and assignment, you need a plugin/programing to manipulate the aircraft datarefs.
i.e. 'the altimeter on the mcp' dararef. Doing a clic you do nothing. You need to store there values 100ft 1000ft 15000ft whatever you need to store that will be the target for the nav systems.
i.e. suppose a 3 position light switch. storing a 0 value means light is off, 1 value means low intensity, 2 means high intensity. So when you build a cockpit, depending on your hardware switch position you store a 0, 1 or 2 on the dataref.
I am giving you just generic examples to understand what a dataref is for
i tried to be as simple as possible on the explanation. Hope this helps