I believe local extensions will be disallowed on stable and beta channel on Windows, according to the blog post. I think this means that on the stable channel on Windows, there will no longer be a “Developer mode” in chrome://extensions. This is because Developer mode is simply a registry setting that malware on Windows can modify, so Chrome will no longer trust this setting.

To develop extensions on Windows, I think you will have to switch to the dev or canary channel http://www.chromium.org/getting-involved/dev-channel

Edit: changed “now” to “will be” for clarity. Thanks Navarr.