running postgresql

I am running Gutsy (Ubuntu 7.10) and new to Postgresql. I followed the
direction at https://help.ubuntu.com/community/PostgreSQL. I executed
the following commands:
    sudo -u postgres createuser -D -A -P myuser
    sudo -u postgres createdb -O myuser mydb
    dagon@DagonUX32:~$ psql mydb

and received the following error message:
    psql: FATAL:  role "dagon" does not exist.

Apparently Postgresql gets confused with my login name. Is this a
Postgresql problem or a Gutsy problem?

How do I establish a "role" that will let me setup different databases?