SYSV Startup Script
Check to see if there is a script called zimbra in your /etc/init.d directory. If there is, depending on your distribution, you may need to run the appropriate startup service configuration program. For instance, in redhat distributions this could be chkconfig. You can use sysv-rc-conf, if installed, to enable the service in debian/ubuntu.
If the script doesn't exist there, manually copy the script:
cp /opt/zimbra/libexec/zimbra /etc/init.d/
Now tell your distribution to start it as described above.