Locally, if you want to access Checkout's DB, first launch Checkout. The welcome window will show, and a loading wheel on the top right will indicate server is being started. Once server started open your preferred GUI Postgres Admin (like pgAdmin or Navicat).

When you setup a new store in Checkout, a 6 character Database Name is used, and non-related to the name you have set.
Checkout comes with a preset demo store...but let's just try to look at which databases are available on the server first.
Set in pgAdmin host as localhost (or any other adequate IP).
set port: 5505
username: admin
password: admin
set maintenance DB the default postgres
you should now be able to login.

Take note of the required DB name, we will need to input this in the module's configuration in Magento. (note that if you did not setup a new store, you should be able to see a DB already there which is the demo store)