Can't clean all cache (master.php)

Hi All,
When i’m trying to clean the all caches the plugin show me this error.
Can’t write to file …../wp-content/w3tc-config/master.php
Could someone tell me how i can fix this issue?
Thanks in advance.

Plugin: http://wordpress.org/support/plugin/w3-total-cache

http://wordpress.org/plugins/w3-total-cache/