Setting up Spamhaus RBLs
Do you have an RBLDNSD daemon set up to serve the data over DNS? You have to do that first before you can do anything else.
Once that is working, you must configure all of the DNS servers that might be used by CanIt to forward all DNS requests for zen.spamhaus.org to that RBLDNSD server. If you don't do that, the DNS requests will leak out to Spamhaus's servers and you'll be blacklisted.
Please contact Spamhaus support for details on how to do the above.
Once that's done, within CanIt, you go to Administration->Master RBLS and make an entry like this:
RBL Domain: zen.spamhaus.org Description: Combined SBL/XBL list from SpamHaus Tag: zen Usage: Block Address Family: IPv4 Type: normal
Then, go to the base realm, default stream and Rules->RBL Rules.
We would recommend seting zen.spamhaus.org to a "Score" action, with a score of 5 and a Greylist Delay of 60 minutes.
Setting up Spamhaus DBLs
To enable Spamhaus' DBL you need to just add these lines to /etc/mail/spamassassin/99_local.cf:
score URIBL_DBL_SPAM 5 score URIBL_DBL_ERROR 0.1