Put the script in the relevant mission directory
OFP/Users/yourname/missions/nameofmission
Then you need a trigger somewhere in the mission that calls the script when it is fired. To do that you need this spell in the activation field of the trigger
[] exec "nameofscript.sqs"
There may be some stuff to go in the square brackets, if there is the script should tell you somewhere, either in text at the top or in a readme. Same with addons. You can open up the script in a text editor like Notepad
Start-Programs-Accessories-Notepad
and see what it says.
If you already know all that stuff and are still having a problem, could you be a bit more specific about what's not working?