In the Beginners Guide to Install Discourse on Ubuntu for Development topic, an admin user should be created by following the step in the Create New Admin section. From your discourse directory run:
RAILS_ENV=development bundle exec rake admin:create
That command will open a prompt that asks for details about the account you are creating. Make sure to enter Y when it asks if you want to grant admin privileges to the account. Once that has been done, you should be able to login with your new admin account.