A lot depends on whether you are using the logical design tools, such as the E/R modeler, or the physical design tool, the database modeler in the Design Editor. If you use the E/R modeler, just draw the entities and relationships. As you add attributes to the entities, you can designate some attributes as being part of a unique and/or primary key.
When you use the Database Design Transformer (DDT) to generate a physical model from this logical model, it can automatically add a surrogate primary key to any entity for which you did not designate one. This key wiil be called, "entity_name"_ID. If you have specified that your database is an Oracle database, DDT will also create a sequence definition for the new table, and associate that sequence with the primary key.
If you start with the physical model, skipping E/R diagrams, you can do the same thing yourself in the Design Editor. There are entries under the table definition for columns, primary keys and unique keys, and separate entries for sequences. Just highlight the header entry, press the "create" button, which looks like a plus sign, and follow the prompts in the wizard. Stick with the Design Editor and the wizards until you've done this a few times. Eventually, you may prefer the property sheets, which you can find in the Design Editor, or the Repository Object Navigator. One of the properties for a column will allow you to specify that the column is to receive its values from a Sequence. HTH.