Newer Visio versions do have down-loadable add-ins as well, but these concentrate on Active Directory documentation, not generic LDAP object documentation. You could write an own add-in to connect to a directory and generate a diagram, if you need a custom solution.
What Visio version? There used to be the "Enterprise Network Tools" addon for older Visio versions, which has been discontinued.
Newer Visio versions do have down-loadable add-ins as well, but these concentrate on Active Directory documentation, not generic LDAP object documentation. You could write an own add-in to connect to a directory and generate a diagram, if you need a custom solution.