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Either drop it onto your closed System folder and it will be installed automatically, or put it in the "Scripting Additions" folder inside your System folder (in earlier versions of the Mac OS the Scripting Additions folder will be found inside the Extensions folder).
Chances are you will already notice several additions already there, installed as part of the original operating system...even AppleScript uses additions to its own commands!
If AppleScript is running you need to quit it and relaunch it. There is no need to restart the computer.
To uninstall a scripting addition, just drag it from the Scripting Additions folder to the trash. The next time you launch your script editor that particular scripting addition will not be present.
To make the Scripting Addition available to only yourself, you should install it into the "ScriptingAdditions" folder in your "Users > UserName > Library" folder. If this folder doesn't exist, you can create it. However, make sure it's spelled "ScriptingAdditions" with no spaces.
To make the Scripting Addition available to all users, you should install it into the "ScriptingAdditions" folder in the root directory's "Library".
For a network, install it into "/Network/Library/ScriptingAdditions/".
For instance, if you run this code, the folder will be created automatically:
path to scripting additions from user domain --> "local domain" or "network domain"
The commands and functionalities of the new osaxen will be available immediatelly.