PostgresSQL: Cannot get to console under different user name
psql assumes you want to connect to the database named as the current user if you don't provide any login details. 

But you can ovverride the defaults using parameters to psql, there is no need to use sudo in order to connect as a different username.

Simply run:

psql -U username -d databasename


where username is the user you want to use and databasename the database you want to connect to.

If you don't know which database you can connect to, use

psql -l


to show them all.

All this and a lot more is nicely documented in the manual: http://www.postgresql.org/docs/current/static/app-psql.html