Yep, this is one of these things when the answer is really easy once you know, but really hard if you don't. I don't know the script in question but from what you've said this should help.
Create a trigger over the drop zone. Now go into Groups mode (F2) and drag a line from the trigger to the chopper. Check out the activation box in the trigger and fix it how you want. In the "On activation" field of the trigger write
release1=1
And that should do the trick. Script starts, chopper carries vehicle, gets to trigger area, trigger fires, variable is set to 1 and drop occurs.
Alternatively, if you want the drop to occur at a waypoint, write release1=1 in the on activation field of the chopper waypoint.
OFP considers any undefined variable to be false. However any expression including the variable (for example a sequence of ANDs or ORs) is also considered to be false. For this reason it is sometimes desirable to define a variable in init.sqs, though in this case it is not necessary. Defining a variable just means writing
variablename=something
where something can be a number, or true or false, or another variable or whatever. As you have probably gathered by now you can write it wherever you want, script file, unit's init field, waypoint etc.
I hope that is all clear and helpful, ask again if not.