Because it operates on the i2b2 database directly rather then using the i2b2 Web Service API, you need a physical connection to that database. This includes network access to the database server (check your firewall when in doubt), a JDBC database driver (for PostgreSQL, it is supplied within the tool) and a database account that was granted the right to write to the required i2b2 project tables. If you used the i2b2 wizard to create a project, you already provided the credentials.
PostgreSQL is by default not accessible from computers other than localhost. That means, if you want to connect to a PostgreSQL database from your local computer using the IMT, and i2b2 and PostgreSQL are running within the VM (or on a server), you need to configure PostgreSQL. First, you need to make PostgreSQL listen on all network interfaces:
Second, you need to specify the host that are allowed to connect to PostgreSQL.
The location of the configuration files varies depending on your platform. For the IDRT VM it is