Trouble with 2.7 installation in add-on site

I’ve been using WP on my main site for a while, but I have an add-on site on the same account, and want to install WP 2.7 on that site. I’ve gone through the 5 minute install steps, and installed WP in a subfolder named “blog” in the add-on site’s folder. Now when I try to access the wp-config.php file, I get the following error.

<Warning: require_once(../wp-load.php) [function.require-once]: failed to open stream: No such file or directory in /home2/mainsite/public_html/add-on.com/blog/wp-admin/install.php on line 18

Fatal error: require_once() [function.require]: Failed opening required ‘../wp-load.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home2/mainsite/public_html/add-on.com/blog/wp-admin/install.php on line 18>

Can anyone help me? Thanks!