I get an error when trying to install W3 Total Cache

I get this error when I try to install W3 Total Cache. I installed it on 2 other websites with no issues, so not sure why this one is causing me problems
This is the message:
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FTP credentials don't allow to write to file /home/(name taken out)/public_html/(name taken out) /wp-config.php

W3 Total Cache Error: Files and directories could not be automatically created to complete the installation.

Please execute commands manually
or use FTP form to allow W3 Total Cache make it automatically.

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It says to add these lines
/** Enable W3 Total Cache */
define('WP_CACHE', true); // Added by W3 Total Cache

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I can find the config file, but it says if I do anything wrong I'll wreck the site. Not excited to give that a shot. Is there some permission I can turn on or off to let the plugin install completely by itself?

If I absolutely must add those lines myself, where am I adding them? beginning, end..???

Please tell me there is an easier, safer, way to do this.