I don't think that issue is (was) connected to the problem with adding repositories.
When I first started PPM, it gave me the "read-only" error and the issue could not be solved from the PPM menu. I closed PPM and started it again by launching ppm.bat as admin (from the right-click context menu), and the issue went away. This way, I can install modules from the Activestate repository. I'm fairly confident that this was just a Win7 permissions issue, although I have to say it's a pretty nasty bug. I'm logged in as admin so it's not that.

Anyway, I only mentioned this bug in passing; the main issue is the inability to add repositories. Anyone have a suggestion?