From the page:
The “filetypes1.text” in the example paragraph is causing me an error of the type “this item does not exist” (Windows)
I copied and pasted the example so I’m afraid there’s something wrong with the example.
You need to add text FileTypes to your project
From the page: “The filter should either be an empty string or the name of a file type as defined in the File Type Sets Editor or via the FileType class.”
Btw Xojo made available for free several books that teach you these things. Might be an idea to download them. See http://documentation.xojo.com