Blogs Directory not working

Hi,
I've downloaded the Blogs Directory plugin and installed it on an up-to-date WordPress MU, and the result is not what I expected. My web server, nginx on CentOS, cannot find the requested resource.

This is one of the logs:
2012/09/28 11:18:45 [error] 16015#0: *4773 "/var/www/blog/blogs/index.php" is not found (2: No such file or directory), client: 172.18.12.5, server: geckos.sis.org.cn, request: "GET /blogs/ HTTP/1.1", host: "geckos.sis.org.cn", referrer: "http://geckos.sis.org.cn/"

Some help would be greatly appreciated.
Yves