TeXShop contains a Macro Editor, so it should never be necessary to directly modify the Macros.plist file.

How do I configure TeXShop?
Modifying the Macros Menu

- Preliminaries
- Recommended Preference Changes
- Obtaining New Library Items
- Converting Graphic Formats Automatically
- TeX Command Line Preferences
- Default Latex Template
- Hidden Preference Items
- .plist Files
- Modifying the Latex Panel
- Modifying Key Bindings
- Redefining Keyboard Menu Shortcuts
- Modifying the Macros Menu