Because a bitmap index is an Oracle specific feature that can only be done in the Physical Model for Oracle RDBMS.
You define the indexes in Relational Model. First go to Table Properties of the table you want to create an index for and press the green plus. Add the information needed.
Then in Physical Model (for Oracle RDBMS) select that index and define it as bitmap index.
When generating the DDL, you will get something like this:
CREATE BITMAP INDEX Orders_Customer_CustNO_IDX ON Orders
Note: If you want ALL your indexes in Oracle RDBMS to be bitmap indexes, you can define a template for indexes and set bitmap as a default. First enable the defaults for Oracle Physical Models Index Templates in Preferences:
Then set (Press View/Edit) the default for Bitmap Index as YES.