Oh, I see now! I was working on Relational Model, those icons are accessible only from Logical Model. I believe I need to spend some more time with Data Modelere. Thanks!
Sorry for opening topic in wrong forum (I haven't noticed the proper one). I'd be grateful if someone could move it where it belongs.
There's an icon in the buttons above the diagram to create a 1:1 Relationship. (The icon looks like 1<-->1 )
Alternatively you can set both the "Source to Target Cardinality" and the "Target to Source Cardinality" properties to 1 in the Properties dialog for the Relationship.
Just to add that there's a forum specifically for Data Modeler issues: SQL Developer Data Modeler