SpamBlocker is included in DirectAdmin. Before activating it you should read and follow the instructions for the version you're running. Unless you've updated yourself, you're probably running SpamBlocker2, and the README is found
here.
Note that if you're already running SpamBlocker2 you probably won't have to install any files; generally DirectAdmin sets up SpamBlocker's required files properly. If you're running an earlier version of SpamBlocker (check the top lines of your exim.conf file) you should upgrade at least to SpamBlocker2; earlier versions are NOT supported.
If you want to test SpamBlocker3 I recommend waiting another day; I should be updating the beta today some time; the newest version has been working on the system hosting my personal domain for several days, and appears to be working properly.
The README information for SpamBlocker3 may be found
here. Depending on how you install it, you may have to add files, and you'll also have to edit the exim.conf file.
In either case you will have to create that web-page mentioned, and fix your exim.conf file so any error messages points to your page, instead of just the generic
example.com (which doesn't really exist). After editing exim.conf be sure to restart exim.
Then simply copy the domain names you host for which you want to try SpamBlocker from /etc/virtual/domains into /etc/virtual/use_rbl_domains.
You do NOT have to restart anything when populating /etc/virtual/use_rbl_domains.
You can monitor what's being blocked by watching the /var/log/exim/rejectlog.
Jeff