Custom Actions can only be use if you have a simConnect client that excepts Payload Strings that can be read by your program.
I would recomend Jim Keir’s Mission Editor for FSX if you want to use custom action. He has payload Strings that can connect to the simconnect SimVars...
IF : Test any SimVar
SET : Set any writeable SimVar
DEBUG : Switch the extension’s debug messages on or off
PROFILE : Set an internal variable
SAVEONFAIL : Save the profile even if the mission fails
FUELLEAK : Start or stop a controlled fuel leak
MESSAGE : Display a small text message on screen
FXTRACK : Attach a visual effect to the player or an AI
METAR : Set local or global weather
WXSTATION : Create a new weather station
EVENT : Send a simulator event
PROFILENAME : Set the name of the file to store the mission profile in
WPT : Set dynamic waypoints relative to the player or an AI
SIMRATE : Set the simulation speed