I take it you are trying to follow the steps outlined here:
UNIX and Windows Accounts in Zimbra LDAP and Zimbra Admin UI 6.0 - Zimbra :: Wiki
In which case you need to install the POSIX extensions so it knows what a "memberUID" attribute is before you attempt to index it...
(Specifically you skipped all of the instructions that you should have followed in
Installing custom ldap schema 6.0 - Zimbra :: Wiki)