Login to your server using FTP, and modify the .htaccess file which is located in the same location where folders like /wp-content/ and /wp-includes/ are located. The easiest thing you can do is to temporarily make the file writeable by changing the permissions to 666. Then repeat the original solution. Don’t forget to change the permissions back to 660. You can also manually add this code in your .htaccess file:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
Another solution: Put index.php at the start of your custom permalink structure, for example:
/index.php/%year%/%monthnum%/%day%/%postname%/