I have entered the descriptions in Data Modeler (Comments in RDBMS), for tables and columns, to be implemented in the database  but when I generate the DDL the comments are not there. What am I doing wrong?

Remember the Preferences? That’s the answer again.

Go to Tools, Preferences.

Select Data Modeler, DDL and enable Generate Comments in RDBMS.

commentsddl

Now that you generate the DDLs they will include the Comments in RDBMS you have entered in Data Modeler:

commentsddl2