Datamodeler is a separate install from SQL*Developer. I do not believe Datamodeler is included in the Oracle 18c client by default.
You would have to download and install DataModeler separately (download from otn.oracle.com).
*** Moderator note: Question now moved to the SQL Developer space as it is not a question related to the SQL or PL/SQL languages.
Its with 12c not with 18c.
DATA MODELER is available by default in SQL DEVELOPER 4.1.2, its missing in 4.1.4
Those are all old releases.
Version 18.2 of SQL Developer and SQL Developer Data Modeler are now available for download on their respective download pages.
SQLDev includes a full copy of the Modeler.