It seems our web host's default security certificate is being loaded instead of our custom one. I've got a ticket open to get the problem resolved.
Meanwhile, you can still use the registration system: click on the "Or you can add an exceptionâ€¦" link at the bottom of Firefox's warning page, then click on the "Add Exception..." button. Click on the "Get Certificate" button in the new window that pops up. Now uncheck the checkbox that says "Permanently store this exception" and click on the "Confirm Security Exception" button.
By following those steps, your browser will allow you to temporarily use the site even though there's a security certificate mismatch. All data will still be transmitted securely and there's no added risk to registering for NDK while the mismatch is in effect. It's just a nuisance.
If you'd be more comfortable waiting for us to resolve this problem before registering for NDK, that's certainly understandable. With any luck we should have this fixed within a day or two. I'll post any updates to this thread.