Posting via the Mac desktop app for 1999.

1. Downloaded
2. Safari automatically unzipped it so I had the 1999 app ready to go in my downloads folder (That's probably browser-specific configuration, but it differed with the instructions so I thought I'd mention it).
3. Dragged the app into /Applications
4. Launched the app and confirmed the security dialog
5. Authenticated with Twitter via the menu on upper right
6. Done!

The only oddity was that after authentication with Twitter, I was briefly returned to the app but then Safari was brought to the front showing an html document from file:///Applications/1999.app/Contents/Resources/app/index.html.

Also, I can't seem to paste into the editor at all. No contextual menu, no Command-v, no app menu. Otherwise, seems to work well. Neat!