If it's a script that you might end up using in all kinds of missions it might be a better idea to put it into a central script folder.
That way you don't have to copy it everytime you write a new mission, and if you should change it you don't have to worry whether you copied it to all the right places. You will also be able to use it in the editor immediately, as soon as you open a map - even if you haven't saved anything yet.
The folder is called \Script, and it should be located in your main OFP directory (where your .EXEs are).
By default it doesn't exist in OFP, so you will probably have to create it yourself.