Custom scripts can be installed in the folder:

"~/Library/Application Scripts/com.codeux.apps.textual/"

This folder might not exist. If it does not, create it. Textual does
not have write permission to this location so it cannot create it on
its own. Scripts installed within this folder benefit from the ability
to be ran outside of the OS X sandbox giving them unrestricted access
to the system as a whole.

Only OS X Mountain Lion or later can read scripts from this folder.
This is an unfortunate design flaw of the OS X sandbox that Textual
cannot circumvent. 

For more information, read the knowledge base:

https://help.codeux.com/textual/Writing-Scripts.kb