Well.. most of that info should be distributed with the script....
the code you have to use to activate the script is there for sure.... and without seeing the script there is no way for me to tell how to activate it.. there are a few common methods... but without seeing the code, there is no way to be sure....
as far as what WP you should place the code in..... You should place it in the one that works the way you want it to ;D
in other words... try them all... and see what result you like best
be forewarned though.... when dealing with waypoints and airborne vehicles, a much larger radius is assigned to the waypoint than when it is on the ground....
meaning that you don't need to come as close to the actual waypoint to set it off....
I've found that most people tend to use triggers when dealing with aircraft as it tends to be more reliable