Permalinks getting 404’s–help?

Hi. I recently started up a blog, and after writing a few posts and setting up some pages (before it was open to the public), decided to change the permalink structure, from the default “?p=N” style to the “Month and Name” style (chosen from the radio button, not hand-tuned). I made sure the .htaccess file was writeable, and it WordPress said it worked, and it did alter my .htaccess file, to look like this:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>

I dont know what this means, but it is not working. When i go back to the blog, the links “look” correct–they say things like https://www.mysite.com/blog/about or https://www.mysite.com/blog/2009/07/test-post.

But trying to click any of these just gives me a 404, and the Apache logs show that the server is trying to GET /blog/about etc., and failing.

Is there something else i need to do to get this to work, or do i need to ahve done this before i started? The site is still not public yet.

Thanks!