If your site is online and you've got access to your controlpanel, you can usually use a FileDirectory function to navigate through your website structure. You need to change the permission settings for the wp-content folder. Now, on a Unix server this is really simple, you can do it via FTP, but on Windows, it's done differently.. annoying, but some say it's safer - that's another debate.
If you navigate to your wp-content folder and get the permissions panel up, you'd need to allow access to users visiting your site. I'm not very good with server configurations, but our host has a site user and a network user: we set the file permissions to read/write for the site user. Save and you're good to go.
If you don't know how to get to the above, just asking your hosting company to tell you how to access file permissions for you site's folders and what the site user is called, should do the trick.