Andrew,
Creating a gdb from Catalog against a Postgres instance in the cloud can be painful, ArcCatalog communicating to the instance and back will be very slow and can eventually time out. Sorry you are incurring frustration. The only thing I would recommend would be to execute the python tool to create a gdb locally on the amazon instance where postgres is installed. You can copy and paste the python syntax right out of documentation.
Sincerely,
Kasia
Andrew,
Have you tried creating the database from PostgreSQL then running the enable geodatabase tool? The enable geodatabase tool creates the repository tables, therefore spatially enabling it.
Hope that helps!