It would be very helpful to know what you are doing. But, in general, you put in the int field of what unit you wish to have the action:
This addAction ["ac1","ac1.sqs"]
Pay heavy attention to the script you make. If your not
formilliar with scripts, browse the tuturials.
ALSO: make sure your mission editor is on advanced, you can toggle easy and advanced modes by the lettering at the top right-hand corner.