I'd like to add a custom toolbox to ArcMap, and also see the script it's written in. To do the latter, I'm trying to open it with my python editor, but it doesn't open it.
To add a custom toolbox, in ArcMap or ArcCatalog, open ArcToolbox and right click in the white space and go to Add Toolbox:
To see the script, right click on the tool, and go to Edit or Export Script. If you use Export Script, create a new text file and select it when exporting.
You might need to go into the Geoprocessing menu, and go to Geoprocessing Options in order to set the default editor: