On the topic of installing an extension in general (Mac guy here) it’s easy IF you’ve got the extension file and it has the right suffix (.i7x). Reading this topic, I’m not sure that ever came up.
So @AmandaB , separate from @rileypb 's advice rounding out the path you were already on – I just want to try to take you back to getting the extension and installing it. It’s low maintenance compared to the path you’re on. It also won’t interfere with the path you’re on if you do both. But with my way, you should end up with the extension available to all your projects, as opposed to just having a copy in the current project:
I see the extension’s on github at this link.
So you go to that page. Then you want to download the file – no copy-pasting! On the top-right of the screen, you’ll see some buttons like this:
Click the ‘download’ button, two to the right of Raw. Pay attention to where on your computer you save it (for this reason, Desktop or Downloads are good options. Easy to find.)
In the Finder, go to where the file is and double-click it.
It should open in Inform, on the nice yellow sheet that tells you it’s an extension, and at the top of the sheet you’ll see an alert saying something like ‘This extension isn’t installed in Inform yet. Do you want to install it?’ Click yes/install.
Now quit Inform, reopen Inform, and you’ve got the extension.
With Inform open, confirm by going to the menu File → Open Installed Extension → Erik Temple → Undo Output Control
Let me know if this series of actions produces this result, because if it does, it shows that you are able to install extensions in general.
Additional pro tip for you:
Right below that ‘Open Installed Extension’ option in the File menu is another saying ‘Show Installed Extensions Folder.’ Choosing that will jump you, in the Finder, right to the installation location of your global extension collection in the Finder. This is in your Library folder, which for you (euphemism:) being a user of the nature you have continuously described as being, and as per @Zed’s fears, is probably hidden in general. So even though a Finder search cannot bring up contents of that folder from a cold start, if you ever want to get to it, you can use that option in Inform.
You can make the Library folder permanently visible/searchable but I’ll only add that if you want/care I doubt it’s affecting your life much or at all.