I successfully added an ldap group to a BOE XI R2 installation yesterday.
In the box where you type in the ldap group I typed cn=groupname,ou=parentldapgroupname,o=toplevelname
I found this hierarchy in the LDAP administrator tool used to create the groups.
When added, BOE XI added "secldap:" to the front of what I typed in so you don't need to type that in.
Naturally you need to have correctly setup the LDAP Server administration credentials and the base ldap distinguished name.