From what I researched, Zentyal uses ejabberd as its jabber service, and it can be configured to store logs via its config files; however, unless 3.4 has changed something since 3.3, I don't believe there is any log functionality in the webadmin interface.
I ended up running Openfire Jabber (which has message archiving and ability to view via web interface) on a separate server, and using Zentyal's LDAP for my user database without issues. If you go this route, remember Zentyal's LDAP is on port 390 and you may have to tweak a bit with the filter settings a bit to get it to separate users and groups properly.