Peter, it sounds like what your asking is where did Chirp save the list on your computer hard drive? The answer is that it depends on where you told Chirp to save it. I don't know the default location, but you can see where Chirp is pointing currently by going to File in the menu bar and clicking "Save As". In the windows that pops up, you'll see "Save in folder" near the top and that will point to the folder on your hard drive where Chirp is going to save the file by default. You can of course change where the file is saved, but I'm guessing you didn't do that.
Then you can cancel out of that Save As dialog and go to File > Open. Then navigate to the folder you noticed in the Save As window and I expect you'll see your file there. You might also see it under File > Recent.
If you don't see the file in that directory - then you might have inadvertently saved it to some other location on your hard drive and you'll just need to go hunting for it (it might be faster to use Finder for that instead of Chirp). Maybe start in your Documents folder. Once you find where the file is, you should be able to Open it in Chirp and you'll have what you need.
Unless I'm misunderstanding something, Chirp hasn't hidden the file from you, but rather it was saved and closed and now you need to tell Chirp where it is on your hard drive - this is how a lot of programs work - Chirp is no different. There isn't just ONE Chirp file, but rather you can use Chirp to open and manage numerous files (both .img files for specific radios or .csv files for storing lists of frequencies not specific to a radio). If you've ever used Microsoft Word or Excel, think of Chirp as program like that and your image and csv files as various documents you open/edit/save/close.
Hope that helps,
Tycen K7TYC