Hi Jerry,
your version of OpenLDAP has been compiled with dynamic backend support (DYNAMIC_BACKENDS=on: Build dynamic backends), meaning slapd is capable to add additional features at runtime (if you use cn=config) or at start time (if configured in slapd.conf). As you can see from the output of slapd -VVV the mdb backend has not been compiled statically into the slapd binary requiring you to instruct it to load the module containing mdb in order to support it. To find out how this is done, you should read chapter 11 of the OpenLDAP Administrators guide (http://www.openldap.org/doc/admin24/backends.html).
You also might want to read the slapd.conf manpage (assuming you still use the plain file configuration), especially the moduleload and modulepath sections. In addition man 'slapd.backends' and 'man slapd-mdb' could be helpful.
Regards,