Can't get w3 Total Cache to work on my site

I have the twenty twelve theme and a child theme running my site. I’ve tried installing w3 Total cache and wp super cache and neither plugin will speed up the site. In fact both slowed down the site! I deleted WP super cache and reinstalled w3 Total cache to try and get it give it another go. The plugin will not complete the install. Gives me messages saying it needs me to manually create directories by logging into my shell. I don’t know how to use the shell program so I created them using FTP but W3 still won’t work. Keeps saying the directories need to be created???