Issues installing plugins from the wpmu dashboard (after updating wp-config)

So I installed wpmu and am trying to install some plugins into my multisite install and I got a message saying that I needed to adda few lines to the wp-config file. So I added those lines and am still getting an error. the exact error is:

Could not create directory. /var/www/vhosts/

My current setup is 755 on wp-content and my ftp credentials should work, so I'm not sure what the deal is. Any ideas? I am using a MediaTemple DV running CentOS with Plesk 11.